I'd say this one: EVGA 7900GS
Or this one: ASUS X1950Pro
They are both rated very highly on Newegg, and this is most likely the cheapest place to get them (you may want to search a little to make sure). Both are going to give you about the same performance as well, and you should be able to max out most of your games at 1024x768. At least for the time being.