a 7900GS is a lot faster than an 8600GT, but it's missing the DX9 functionality. Most future games will be able to play in DX9 mode. There are no real main aesthetical differences between DX9 and DX10. Someone once described it like this: "DX10 has pretty much the same functionality as DX9, but it can do a lot more of those functions and more efficiently"
Look at it this way, PS3's running on a slightly modified version of DX9 (same goes for other current consoles) and it can still manage something like Killzone 2, which, visually at least, (and this is arguable), can be comparable to a DX10 title like Crysis, judging from a few HD videos available online. However, running at 720p is just about running something at 1280 x 720 (I think) and it runs @ 30 fps, which ain't really impressive by computer standards. It also depends on art direction too, for a game at least.
Anyway, whole rant aside, I'd go with a 7900GS or really put a little extra cash and get an 8800GTS which'll last you a **** of a lot longer. The 8600s and ATI's X2Ks are bread & butter market gimmicks. Last generations cards high-end cards, which are about the same price, will do a better job than the mid-range cards of this generation.
