http://www.tomshardware.com/reviews/ati-agp-3850-agp,1939.html
They tested an Athlon 64 like yours with newer cards to see if there was a bottleneck. For general 3D games, the 2600XT was the limit that the CPU didn't bottleneck. Therefore, you should buy whatever is the cheapest out of the following, all similar in performance to the 2600XT. Oh - make sure your board supports PCI-E. If it doesn't, newer video cards WILL NOT WORK. If you board is PCI, you have no chance. If your board has AGP, there are very few options but the 2600XT is one of them.
Here are my suggestions - pick the cheapest. The 8800GS will be no faster than these due to the CPU bottleneck.
Radeon HD 2600 XT
Radeon HD 3650
Radeon HD 3670
Radeon HD 4550
Geforce 8600GT
Geforce 9500GT







