The new Windows OS, Vista, is going to champion in DirectX 10. That's kind of like the base of what all games use to play. Right now, the vast majority of cards are DirectX 9, which is fine, because nothing is using DX10. But future games will. Ones available now are $400+, which is just too rich for my taste.
The other end of the spectrum is that Intel is going to release their quad core CPUs soon, so that means that their brand spankin' new and already affordable Duo 2 Core CPUs are going to drop in price.
So really, just wait until Vista rolls out, the DX10 mid-level cards to hit and the quad core price cuts, and you can get a nice rig built for the future at a good price.