Just agreeing with others on Zalman, best money spent for O/C'ing.
You didn't mention which type of 6600 you are running: PCIe, or AGP.
I have the AGP, and the Zalman overhangs the edges of the board a little, just be sure yuou don't let it contact anything you wanna keep. Also, you have to be extra careful rummaging around in side the case afterwards, so as to not bump it in it's mounts. The more overhang, the more leverage you place on the mounts . . .ie: bend / break.
Xblade case/450W PSU/GigaK8NS M/B/AMD 3000+ -939/Leadtek 6600gt-AGP/Corsair XMS TwinX 1G DDR2: AquaMark:53,553/3DMark05:4,240 = GT idle@42'C/57'C load - 540/1125 clk spds