Get the Nvidia 8800gt with 512mb of memory, it's the best card from a price/performance point of view, it's about 200-250$, better than that you have the 8800gts but you only gain 2-3 fps in most games and it costs way more...same for the top range 8800gtx which is older and in some new games is even slower then the 8800gt.

And of course another gb of memory, they're dirty cheap now.