Depends on the game mostly, some game look better in dx10 than in dx9, now about the difference between those cards, well the 7900gs has a little better performance but then again is almost obsolete
Agree, but at least you have good choices unlike some people like me jk, i would go for the 8600gts since the only 7900gs belongs to the crappy foxconn
Go for the 7900GS, it has a higher cycle rate then the 8600. (128/256) Also, the 8600 is designed for DX10, while the 7900GS would suit you because XP is limited to DX9. (for the most part)