Originally posted by: Broadkipa
I play online: bf1942, CS Source and off line doom3, those are my faves at the moment.
BF1942 will run much better with a 6800GT, but you're probably at a fairly acceptable resolution and video option level with the 5900XT already, as 1942 is fairly old, and not terribly demanding. If you're at all interested in the upcoming BF2, I would assume a 6600GT a minimum to get decent performance. In BFV my 5900XT barely ran acceptably at 1024x768, I imagine BF2 will be more of the same (using lots of DX9, the primary weakness of the 5xxx series)
CS Source should see an improvement in quality (DX9 vs. DX8) plus some frames, but is fairly CPU intensive and may also be limited by your memory bandwidth. Doom3 should see decent improvement in FPS, as you can tell from the link that nemesismk2 posted, but it's hard to tell if there would be any CPU/mem bottlenecks.
Personally, for a system like that, I'd probably save up for the eventual full system upgrade that you'll need to go to DDR and PCIe. You'll definitely get some benefit from a video card upgrade, but I don't know how much I'd spend on an infrastructure that old.