the 8600 range is superior to the 7900 range and the 7950 range. the 8600 memory core runs a 2000mhz wheres the 7950 only goes up to about 1600mhz or so. The gfx core itself is also much quicker off the top of my head around 100mhz or so faster than the superclocked 7950GT.

The 8xxx range of nvidia cards are just pure class at the moment but I am willing to bet that the 8600 benchmarks will be much better than the 7950 benchmarks