@timmeh87: I'm having the exact same problem but with an eVGA 670 (FTW 4GB). I tried the "Enable VGA Palette Snooping" but didn't work for me. I've tried what you tried and got the same results, the card works in another machine and the onboard VGA works fine. The monitor kicks on like it's going to work but then I get the "no signal" on the monitor. I've flashed both mobo and 670 BIOS to no avail.
@DrItanium: any suggestions as to what else to try in the BIOS?
I'm running 2 2687w's with 64GB RAM and twin H80's, Corsair ATX1200 PSU.