clocks don't matter, my old 9600XT ran up to ~650MHz on core...
DX10 means diddly squat too, it offers no visual improvement over DX9 and is only available with Vista, a poorly optimized and buggy operating system. It has no other features that really distinguish it from DX9... it has offered absolutely nothing new, and already we're hearing decent talk about DX11
In fact, DX9 is more of a bonus because you get the same visuals with generally better performance.
The 8600GT won't be even capable of utilizing 512MB of vram, nevermind 1GB
Arguably, 256MB is plenty for anything running at a resolution equal to or under 1680x1050
512+ is really only needed when you're playing texture-heavy games at resolutions beyond 1680x1050, and you need a card that is architecturally capable of efficiently being able to put that memory to use... it's like putting $1500 22-inch performance wheels on a 1998 Hyundai accent with 300K mileage...
performance-wise, there's almost no difference between the 7600GT and 8600GT
trust me on this, most people on this forum will agree with 100% of what I'm saying