Be carefull with those GS Models of the Cards, they're dowgraded Versions of the GTs, mostly they've even cheaper Performance then the GTs from the previos Cards (8800 GS < 8600 GT) sometimes even lower Performance then the previos Generation (7000er series).
In most cases they reduced not only the clock speed, but also the shadder units and pixel pipelines and of course the Memory to get it as cheap as possible.
Maybe it would be better to spend a few Dollars more to get a GT or GTX.
However it depends on what you wanna do with it, primary playing Games like Crysis or just running Applications.
Well, if you didn't want to play Games you would be satisfied with your current Card right?
A big plus to the Performance could also be, to add more RAM to the System (max. 3GB @ XP), and the Prices aren't that high ATM.
But I think you should post your specs first, before anyone can say what's the best for your System.