The 7900GT, 7950GT, etc are much better then the 8600GT and 8600GTS.
The reason people are going with the 8600 instead is because it's a DX10 card, and they want to have newer technology. Although in reality it's pretty stupid because as i've said numerous times, the 8600 series won't be able to play DX10 games on high settings even. So comparing a DX10 game on medium to a DX9 game using a 7900GT on high settings, the graphics would probably be worse for the 8600.