games take advantage of the pci express
i dont know if you WANT to spend that much on the vid cart, but you can get a 6600gt for like 110$, which is a pci-e card and its pretty good....or a 6800gt for 269$.........i dont think you HAVE to get ddr2, and if you arent worried about over clocking, you can get some value corsair ram (1 gig) for 99 dollars.... and dual channel that....mobo's, depending on how much you want to spend, plus if you are going stock or oc'ing, you can spend under 80$ on a good pci-e capable card..
so basically you could get away with like 300$ for mobo+ram+vid card, and have a pretty good system running.