After extensive research, I've learned the pro's and the cons of a Dual Core processor.
First, while obviously the future is heading towards dual cores, it is important to know that currently, having a dual core does not make you 'faster' at everything your processor can see.
Essentially, it is dividing the processor into two, so that it can multi-task more easily. However, if you are not multi-tasking, you may actually be losing some processor speed. Single operations may gain from having a single core rather than a dual core if they are in a comparable range. However, obviously newer dual cores, even though their resources are divided are still faster than many single cores.
Intel has already announced their quad core and AMD's is just around the corner. So, multiple cores are definitely the future. With that being said, if all you do is Outlook express with a touch of word on the side, I wouldn't feel the pressure to upgrade.
However, if you like to encrypt and decrypt documents with a touch of video compression on the side, all while you're playing your favorite video game on another monitor, What are you waiting for!?
It is also important to note that many newer software applications are coming coded for dual core processors, the term would be to 'thread' the coding. This basically makes it so that two cores can work on one project at a time, which is faster than only one, though, as mentioned above, this is only seen in some of the newest software. Windows Vista, however, is going to be one of these new applications coded for dual core processors.
So the question ultimately boils down to how much money you want to spend and how long it is you plan to wait until you upgrade to Vista.