Well, depends on what you want.
8600GTS is DX10 compatable, meaning if you're switching to vista, you dont have to worry or spend money that much sooner if more DX10-only games comes around the corner.
7900GS is one of the best GPUs in the DX9 gen, have all of the things that makes it a better cards for XP. It can support most of the games on the market now, but if you're going to switch to Vista, then just choose 8600GTS.
Be mindful that your computer determinds what GPU you can use/buy. Both cards needs a maximum of 82W of power to operate(that means u need abt 85W for them to work for safety), also both cards excel in different departments in performance. 8600GTS is more "ATI" than 7900GS, meaning it does better than 7900GS in shading, but still have the cool texturing ability of its grampas.
I got this page which would help you alot, i visit it alot also, hopes this helps you.
http://www.gpureview.com/show_cards.php?PHPSESSID=748230910a1ab65c3b53a19ea4913ac8&card1=443&card2=512