You could try what HeadShot did, clear the CMOS, and start from scratch when he flashed his BIOS to F4. I think he also set his RAM timings manually. I decided to start from scratch as well and see what I can come up with, since I also need a high vcore compared to most to get 400x8 stable.

It could very well be that your board and the 6600 aren't cooperating nicely like tends to happen with the Allendale chips.