Well you can play most games well, but many newer games, like Battlefield 2, will run better with 1 gig that 512 mb. Ram isn't that expensive, about $100 for good dual channel 1 gig set. By the way, good choice on the AMD, they kill the P4s in gaming and calculation-intensive apps
. Also, i would suggest you get a 6600gt instead of a 6600 because the GPU is much more powerfull, even though it has less Ram. Maybe you should wait up, because the new Radeon X1600 series looks pretty good, and maybe isn't showing its best since ATI probably didn't have enough time to optimize the drivers for games ect yet so I expect more from the R520s. Just my suggestion here, the choice is yours. By the way, look for Kingston Hyper-X dual channel ram, its very cheap and has faster timings than other Ram of its price. 2-3-2 timings are near best performance, so you shouldn't worry about the speed of that Ram.
Athlon 64 X2 4400+, nForce 4 Ultra chipset, 1 gig ddr400 d-channel, GeForce 7800 GTX OC,2 WD 74 gig Raptors in RAID