Personally I'd advise building a system with upgrades in mind. The Mobo and RAM are my primary concerns though RAM is so cheap now as to be insignificant.
I recently built a system using a M2NPV-VM mobo and put a Sempron 3200 and 2 gig into it. It has an integrated gpu (Gforce 6150) which frankly will only just run the most intensive games, there are a few out there which may be a bit beyond it. However once the high end express video cards come down in price to a reasonable level its an easy and cheap upgrade. Same with the processor, very little out there requires a faster cpu than the Sempron so I'll wait until the dual core Athlons are cheap and bang one in.
You pay a big premium for the latest kit, I find it to be better to buy a good quality mobo with lots of upgrade potential. total cost to upgrade was ~ £130. Total cost of putting all the latest stuff in the same mobo would be somewhere above £400. Upgrading again in a year or so will probably set me back another £70.
I doubt the above system would necessarily suit your needs but you get the idea. I don't really play games anymore though my system would happily run 90% of those currently on the market. The only computer I've ever bought in order to run a specific game was tailored to Frontier. Imagine my suprise when it came on a single floppy disk...
