I would also get 8600GTS over X1950Pro for several reasons
1- X1950Pro may get better FPS in most games, but 8600GTS is showing its potential in newer games like STALKER, Oblivion, Supreme Commander. In those newer games 8600GTS is performing as good as X1950Pro or even better.
2- 8600GTS produce better image quality, and I think having better image quality is better than getting a little extra FPS
3- 8600GTS consume less power and produce lower heat than X1950Pro
You can see a review for 8600GTS and X1950Pro from here
Nvidia's GeForce 8600 series graphics cards - The Tech Report - Page 5
Since 8600GTS and X1950Pro have similar price, I would recommend 8600GTS