go for the 7950, it will beat any DX10 card except the 8800 series. Here you go, you can compare the charts to look at the performance of the 7950, they don't list the 8600's, so here is a review with them:
charts
review
I cut the the middle of the review because 3Dmark really doesn't matter too much with the newer video cards. The 8600GTS's appear to beat the 7950 in every 3DMark benchmark, however they never beat the 7950 in any of the games tested. You might be able to if you overclocked, but overclocking is a gamble, you might get a card that doesn't overclock a single mhz, you might not. Up to you.
Also, Dx10 requires massive power, the 8600's really can muster that kind of power at any reasonable resolution or AA/AF level.