Just overall performance.
The 7600 GT is better than the 7600 GS. Both are fine cards.
My current computer is probably 50% of the computer you are looking at's speed and runs oblivion medium-low graphics. So be expecting medium-high, high, or very high. Probably high or very high.
The average 7600 GT/GS card has 256 MB of RAM. That one has 512 MB. Yes it does help to have more RAM, there is no denying that. BUT you have to have the video card to push 512 MB of RAM. The 7600 GS isn't going to be able to use all 512 MB at anytime. So you're better off getting a 256 MB 7600 GT for nearly the same price.
If you wanted to put it this way..
Let's say there are two racecars.
A slow one that has a large gas tank
And a fast one that has a normal gas tank. And they are racing. The one with the small gas tank can use that gas well and to it's full extent, thus meaning it will win.
The fast one is the 7600 GT and the slow one is the 7600 GS 512 MB. You want the one that wins. (And no it doesn't cost money for gas bills

)