well, do you have pci-e or just agp? if pci-e i would say maybe a 4670 or 4830 (just incase you game even just a little) if agp the best i know of is a 3850. we need a little more info like how much can you spend or want to spend and what can your mobo handle.(pci-e or agp)
You might want to look at doing a complete upgrade since now your going to spend alot of money on old ram and a AGP card while for the same money you could be getting parts that are way better. It would not even have to cost that much, depending what parts you have that are still useable like the drive case and psu it would not have to cost more than 500 for a system that would way better and is a way better investment.