8600 Gt > 7900 GsIn response to original post, it depends on what you mean by which is better. One has DX10, the other doesn't. One is newer and less expensive, but doesn't run games quite as well.
Because I run the 8600M GT in my Asus G1S-A1 notebook, I will discuss that card as a better comparison with the 7900 GS. I consistently get 4000 3dmarks in 3dmark06. My friend has a desktop computer with the 7900GS and he scores lower than I do in benchmarks (though not by a lot). That link http://www.notebookcheck.com/Mobile-...ste.735.0.html is fine, but don't take it to be the exact ranking of how a card is compared to other cards. For what I have seen in actual performance, the 8600 GT outperforms the 7900 GS.
My brother runs the desktop version of the XFX 8600 GT (DDR3, just like my mobile version). He picked it up for $109 off of zipzoomfly, and it came with a free copy of Company of Heroes! To me, that makes its value much better than that of the 7900 GS, which costs $70 more. I will ask him for his benchmark scores.
And as for real game performance, I thought that my comp would be able to play CoH better at 1920x1200 resolutions, so I was a little sad to see that I had to turn some things down to get 30-50fps. But then I saw how my friend's 8800 GTS did in CoH and we were running the same settings and resolutions and the game looked about the same. So I'm plenty happy with the performance, and I would never go back to a 7900 card.
I don't know anyone running dual SLI 8600 GTs, so I can't say about that, but the single 8600 GTs run great when compared to the 7900 GS. For now, when the 8600 GT is one of the top cards to fit in a 15" notebook, I'm plenty happy with it.
That is my humble opinion,