I just put together a system with a Gigabyte EP45T-DS3R and a 8800GTS and had all kinds of issues. It would freeze in the BIOS screen occasionally and any video game or 3DMark06 action and it would freeze almost instantly. I was on the original BIOS (F1) and it all got fixed with the F2 BIOS. There is a beta bios availible but I did not try it. Are you using the newest full BIOS or a beta? Try different ones if availible.
I hope you get this sorted it can be real frustrating, I feel for you. Good luck!
Edit - For some reason I did not see your above post. Glad you got it sorted
ASUS P5Q-E - E8400 (4GHz) - Corsair TX650W - 2xHIS HD 4850 IceQ Turbo - 2x2GB Mushkin Ascent 8000 - 2xWD 500GB - TRUE 120 - Antec 1200 - ASUS Xonar DX - Big Smile