6800 maybe be a good card for the price, but a 6800GS is not as good as a 6800, and a 6800GT would be even better. Heck, I say get that new AGP version of the 7600GS. It'll cost around $120-160 if you get it from NewEgg.com. Make sure it's AGP slot, NOT PCI-E slot, and make sure you HAVE a AGP slot to begin with!
If you have a AGP slot, do yourself some good and don't get a (nVidia)5200, (nVidia)6200, (ATi)9250, (ATi)9000, (ATi)X300, or (ATi)X1300 of any type because they are budget cards and won't do you very much for you. I still say, get a 7600GS or 6800GT, but if you really want go ATi, get a X1600 or a X800XT, just always make sure that it is a AGP version if you have a AGP slot, or a PCI-Express version if you have PCI-Express slot(s). And always check NewEgg.com and compare their prices with everyone else before you buy from someone.