First, if you don't have a Vista machine then DX10 isn't going to be an issue.

Without giving too much away, Conitec is looking at DX10 but it's really hard to say if and when we'll add it to A7.

IMHO: If you plan on using your PC for the next 2-3 years, and you are interested in using Vista, then I would look at the 8600. But, if you plan to buy a new Vista PC in that time, the 7900 is probably all the power you'll ever need for XP development.

BTW: I am NOT a hardware guy (I had to look up the specs on the 8600 and 7900 before I wrote this ). So don't take my word for it.