Many people will tell you to make sure your computer will recognize PC133 memory before you buy it. This is a good warning but, doesn't describe the REAL problem. The following explains the actual problem and what you can do about it.
(31 July 2006)
Having worked in computer DRAM memory hardware development and testing for one of the larger computer manufacturers in the 1970's and 1980's, I have a different perspective of what is happening.
When PC133 memory first came out, it was simply a speed upgrade of the existing PC100 DIMMs (Dual Inline Memory Modules) and they worked together without exception. As the memory manufactures became more adept at making SDRAM, they found that the RAM didn't need to be refreshed as often or, they were able to make it more cheaply by increasing the number of refresh cycles required to refresh all the memory cells on the chips.
These cells are in reality "leaky" capacitors that hold a charge depending on weather the bit is a "zero" or a "one". The original DRAM (DIP) refresh was done in 128 or 256 RAS-only refresh cycles, every 4 milliseconds to keep the capacitors charged. Most of the PC100 SDRAM DIMMs use 2k or 4k refresh cycles on a different timing schedule. The newer PC133 DIMMs require double or quadruple the number of PC100 refresh cycles be used to completely refresh the memory chips on each DIMM.
If you plug a newer PC133 DIMM into an older machine that was designed for PC100 memory, because it is being refreshed for only half (or one quarter) the required number of refresh cycles, you will only "see" part of the memory.
There is a ROM on each DIMM that identifies it's properties to the system and this causes a problem when plugging in two PC133 DIMMs that are only partially refreshed. The wasted memory space of the first DIMM is reserved because it has told the system that it has more memory than the system "sees". There is now a "hole" in memory between the first and the second DIMM and the system doesn't recognize the second DIMM because the memory is not continuous.
What can you do?
First, KNOW THE NUMBER OF REFRESH CYCLES YOUR MOTHERBOARD SUPPLIES! This is dependant on the chip set on you motherboard and the chip set initialization code in you BIOS. (Theoretically, depending on the limitations of your motherboard chip set hardware, the BIOS initialization code could be changed to use a higher refresh number allowing all available DIMMs to work together. I have never done this so, it is just a theory.)
Second, ASK THE VENDOR (OR MANUFACTURER) HOW MANY REFRESH CYCLES ARE REQUIRED FOR THE DIMMs YOU ARE CONSIDERING PURCHASING! This will tell you immediately if the memory will work in your system or not. There is no magic about it. Ask if the memory requires 1k, 2k, 4k, 8k, or 16k memory refresh cycles.
If the number of refresh cycles required by the DIMM is the same or less than the number that your motherboard supplies, the memory will work just fine. If it is more than your motherboard supplies, you will only "see" part of the memory on the DIMM. Additionally, any other DIMMs in the system may not be recognized at all. No Good!!!
CONCLUSION:Always keep the number of memory refresh cycles required for your computer memory in mind when you expand your main system memory (RAM) and you will have no problems selecting compatible memory for your computer.
In addition, don't try to put more memory on your motherboard than it is designed to handle or use DIMMs that are larger than the motherboard is equipped to address. Look at the motherboard manufacturer's web site (in the "motherboard manual") to find your motherboard's exact specifications.*------------------------------------------------------------------------------------
ADDENDUM #1: (10 August 2006)
CHIP DENSITY (LOW vs. HIGH)
Think of the insides of each memory chip as an X-Y matrix of (for instance) 4096 by 4096 capacitors of less than 1 picofarad each (16 million bits of data). These capacitors are the actual memory elements inside the DIMM’s memory chips. The cells are address decoded by rows and columns to single out an individual cell for each address. In order to refresh the leaky capacitors in each array, a row address is applied to the array and the RAS line (Row-Address-Strobe) is clocked to refresh one entire row of the array. In order to refresh the entire array you must do this to 4096 sequential addresses to cover all the rows (4k-refresh). The column address is not used during the refresh cycle. It is used only to single out a cell for read or write cycles. These refresh cycles occur between uses of the memory and are delayed a small amount when a CPU or DMA (Direct-Memory-Access) operation is in progress.
Older memory chips (low density chips) were designed with fewer cells and therefore used a lower number of addresses to select all the rows. When larger arrays of cells were designed in the newer chips (high density chips), more row address lines were used to address the larger arrays of cells and these chips require more refresh cycles to refresh all the rows of capacitors in the chip.
What has been interpreted, as a difference between low-density and high-density chips is really the problem of required refresh cycles and not the density of the cells themselves. Refresh is the ROOT cause of most DIMM incompatibility. If the chip designers had kept the same number of rows of cells in the "high-density" chips and just increased the number of columns, then the chips would still require the same number of refresh cycles and they would work in all systems. The problems with just adding more columns are that there would be extra propagation delays (slower memory) and serious power requirement problems that cannot be easily overcome without adding more rows of cells. Your power supply will not tollerate the eventual 70+ amp surges on the +3.3 volt buss.
If your system only sees ½ or ¼ of a DIMM, then a refresh incompatibility is the root problem. If you know the number of refresh cycles required by the DIMM and the number of refresh cycles provided by your motherboard, you can avoid this major roadblock to memory selection. The problem is we can’t see inside the chips so, asking the manufacturers is the only way to be sure.
I am currently successfully using a PNY PC133 256Mb DIMM (I was given for free) in my old Abit BH6 motherboard, 866Mhz PIII computer. Because this DIMM requires 8k memory refresh cycles (it says so on the package it came in) and my motherboard only provides 4k memory refresh cycles, I can only "see" 128Mb of the RAM. The rest of the RAM is "dead" to the computer because the charge on 1/2 of the memory cells leaks away (without adequate refresh) and therefore it is not recognized by the system. I cannot add any more memory to the system in the present configuration. The only change I can make is to replace the DIMM with a 4k-refresh part and the problems will go away.
There are other reasons why some DIMMs won’t work in some systems. All PC100 and PC133 DIMMs have a ROM (Read-Only-Memory) onboard that informs the system what the characteristics of the DIMM are. If the motherboard is not set up to handle these characteristics, the system will not work.
Buffered or registered DIMMs have an extra circuit between the memory output and the CPU so there is a delay in signal propagation. The CPU and motherboard chip set waits a fixed amount of time to read the data lines from the DIMM and, because of the buffer delay, the CPU may grab the memory data bus before it has settled. This yields bad data reads from memory and the motherboard refuses to work with that DIMM if the motherboard is not set up to work with it.
Another problem is when the motherboard chip set detects ECC (Error-Correction-Code) memory bits. If the motherboard chip set is not set up to work with ECC bits, it can refuse to work with that DIMM. Some motherboards just ignore the ECC bits.
Lastly, each chip manufacturer designs their chips to require a minimum loading on the address lines and supply only so much current on the data output lines. Most manufacturers try to meet a common standard. Each batch of chips that are manufactured have slightly different characteristics that sometimes might not quite meet your motherboard's signal requirements and therefore don't work. This can happen with any manufacturer. The parts meet all the testing specifications but, they won't work in the final system.
ADDENDUM #2: (25 August 2006)
SYSTEM UPGRADE
I replaced the single 8k refresh, 256Mb DIMM on my Abit BH6 motherboard with two 4k refresh, 256Mb DIMMs and I am now running sucessfully with 512Mb of RAM. Another problem I ran into is that some older motherboards are not set up to use more than 512Mb or 768Mb of RAM. This has to do with the layout design of the bank select lines on the motherboard, not the DIMMs themselves. There is no way around this problem.
The maximum size DIMM that a motherboard can accomodate is different for each motherboard. My Abit BH6, R1.01 motherboard was originally designed to use three 128Mb DIMMs but, it can handle a maximum of only two 256Mb DIMMs.
My motherboard required me to plug the 256Mb DIMMs in sockets 1 and 3, leaving the middle socket empty. I am now happily running with 512 MB and I will not try to add any more memory to this system. My only option is to upgrade to a newer, faster computer with a larger memory system in the near future.
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ADDENDUM #3: (20 September 2006)
FYI: Changing the Motherboard Refresh
I looked up the information about my motherboard's Intel BX chip set and found that to change my motherboard from providing 4k refresh to providing 8k refresh, it is simply a matter of reversing 2 bits in the BIOS initialization code. Some day I may try to change the code just for fun but, I don't have the time or the physical space to start another project right now.
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ADDENDUM #4: (28 July 2007)During the last 6 months, I have received reports of several sales people and so called "tech reps" or "support engineers" telling customers that they know nothing about the refresh rate and misleading customers by stating that you don't need this information. Disturbed by these reports, I spoke with the hardware design engineers at several of these companies and confirmed that the refresh compatibility information requirement is still valid.
The real story is that many companies are ignoring the problem by just publishing a memory-motherboard compatibility list and not informing their sales people about the ROOT problem. PC133 memory is dropping in sales and the manufacturers are mostly cutting off support for this older RAM technology. If you persevere and insist that the motherboard and RAM manufacturers give you the information you need, you will win!
As I mentioned in Addendum #1, some RAM manufacturers already publish this information and it appears on their retail packages. If it is convenient, check the package at any local store that sells RAM or ask the vendor to check the package.
IN GENERAL: 4K memory refresh PC133 DIMMs are usually PC100 compatible. Conversely, 8K (and higher) memory refresh PC133 DIMMs are not PC100 compatible.

RAM Compatibility for Laptop Image