Well, from what i have read, imo, you would be better off trying to get a 7900gs(only $105 after rebate from newegg), from every almost every review i have read, they will beat a 8600gt, and draw even or just slightly behind a 8600gts, and with overclocking it will almost always beat it
if you are planning to get a 8600 to try and future proof for dx10, just let me say that i doubt that these cards will be able to give you dx10 goodness at a resonable res anyway
imho if you want to overclock you couldnt go wrong with a 7900gs, they oc like crazy
but if you really want a dx10 card, then definately save a little more and get an 8800gts, or you could wait for the next generation of dx10 cards, now enough with my opinions, i'll now try to answer your questions that havnt been answered
Pros(of both): DX10
Future proofed(in a way)
Cons: a bit underpowered for a "next gen" card
you should expect a fairly large performace difference
same goes with gaming
i had to face this exact same choice about a month ago when my
mobo blew up, you can see what i chose
btw, when ocing a 7900gs i can get to over 30% performance increase
bottom line: if you need one now, i would recommend a 7900gs, if you want dx10 then the 8600gts
but if you can wait then just wait for the secon gen of dx10 cards, save some money and byy that time you can get an upper range card
Hope i didnt just make your choice more confusing, good luck