The GS is the weakest of the 8800 line... slowest core & memory speed, lowest memory bandwidth, etc.
I've have a 640MB GTS (second slowest of the line) for over a year now and I can play most games in med- to high-quality details at high resolutions (1600x1200+) without trouble.  Although note that I don't play too many first person shooter games.
In any case, don't forget that Nvidia's current line is the 9-series... so you'll be buying old technology. 
At this point, if graphics are important to you and you want your new graphic card to last you longer, I recommend getting something with a 9800 card.  Also keep in mind that buying newer generation video cards might NOT speed up old games, but will likely be faster with new and upcoming ones.