The 8600GTS is somewhere between the 7900GS/7900GT in performance, BUT it has DX10, wheras the other cards don't - If you want to play a DirectX 10 game in DX10 mode you WILL need Vista and a DX10 compatible card - which is any 8-series from nVidia or any 2-series from ATi. It's up to you as to whether you want to get DX10, because so far in the DX10 games even 8800GTX's are getting pretty crappy performance, but the games themselves were quickly (and sloppily) ported to DX10, and so we'll see "true" DX10 performance with stuff like Crysis.
An x1950 Pro is probably the best "bang for the buck" right now, as it sits somewhere between the 7900GS/GT and is nice and cheap, but as i'm an nVidia fanboy i'd recommend also looking at the price of a 7900GTO on eBay, as they are really rather quick and now rather cheap, too.
Personally i'd see if i could afford an 8600GTS
/me remembers about 9 months ago when i bought my 7950GT for £186 - just seen an identical card (but with more RAM) sell for £67