In general, the 8600gts is better than the 8600gt. I suppose if you use really high resolutions, the 8600gt might outperform the 8600gts.
But if your resolution is less than 1920X1200, then I think the 8600gts will be the better card for you. Especially since it is DDR3 and the 8600gt has DDR2.
I'm not completely sure, as I haven't seen any benchmarks, but that is what I would go with.
You can always try them both and do benchmarks yourself, and see which one performs better.
EDIT: I meant that I am sure that the 8600gts is better, but I'm not sure at what resolution or if there is a resolution where the 8600gt might be better because of the ram.