...a smarter thing to do would be to sell my 7900GT and buy 2 8600GTS...
That's the stupidest thing to do!
8600Series graphics cards suck!
Why you want to go with SLI?
Anyway, i would go for a bigger case+8800GTS 320MB if i were you!
I agree on all points there.
8600 = Abysmal.
SLI = Pointless ATM
Sell the 7900GT and get a proper case and an 8800GTS.