I am getting ready to build a new one and am going to use a quad core Intel.  More and more applications are being built to use multiple cores and even with a little slower speed per core the extra cores make it faster.  To go above 3gig of RAM you will really need to run Vista 64bit to handle the RAM.  If you have apps with lots of maps and pictures I would look at a bigger HDD, at least 500 or even bigger as cheap as they are getting.

Gunner