Don`t buy any GTS with 320 or 640mb. Its old now, and compared to the 2nd version(GT) uses more electricity(sounds daft, but if its on 24/7 you`ll notice)
8600 is crap.
Its up to you at the end of the day, personally, I`d wait for the 9*** series, its not going to be long, or look at Radeon 3850/3870(Not as good as 8800GT though).
This should help http://www.tomshardware.co.uk/graphics-car...ew-29792-6.html.
As Festo said, make sure your PSU has enough amps/watts to deliver.