Depends on what kind of games you are wanting to play. If you absolutely have to upgrade now, and don't plan on playing the latest and greatest, the 7600GT that RadiclDreamer provided a link for would be your best choice. You could get a 7900GS for 160-ish AR, that is beefier than the 7600, but your processor would prolly become a bottleneck then and you wouldn't be getting the full potential out of the card.
If you're an ATI fan, take a look at the X1950PRO. It will also beat the 7600GT in performance, but also requires more power to run it, and again you would probably become cpu limited with it.
Correct me if I'm wrong, but your motherboard is a socket 754, right? If so, then if I were you, I would hold on to the money you have saved now....try and sell current rig and/or save some more...then start fresh, upgrading to a newer socket and pci-express. I'm just sayin
Soooo yeah, given your system specs I'd say 7600GT would be the best balance. But if want a little more beef to play some newer games (just not the absolute latest and greatest) then look at the 7900GS, or X1950PRO.
Hope this helps!