Slightly? overclock them both and the 7900GS beats the X1950PRO which the 8600GTS is slower than. The lack of DX10 won't matter much when by that time neither of those cards will be able to handle DX10 games very well I think. Go for whats faster now and then sell the 7900GS later and get like an 8800GTS if you really want DX10.
First: 8600GTS can beat X1950pro in many games like STALKER and oblivion, so I say that they are pretty equal in term of performance.
Second: I was comparing stock 7900GS to stock 8600GT
Third: If you want to talk about overclocking then both 8600GT and GTS can be overclocked at very high speed.
In this website they were able to overclock 8600GTS core to 1GHz !!! (But you need to voltmode it)
VR-Zone : Technology Beats - NVIDIA 8600GTS Overclocking!