Originally Posted by QuaziBee
Make sure you plugged your 4pin power cable into the mobo as well as the PCIe power.
I was going to say the same thing. Make sure you've supplied the external power required for most new video cards. That's in addition to plugging them into the PCI-e slots. The slots can't provide enough power. (On my latest graphics card, MB and P/S it's a 6-pin connector on the graphics card, btw.)
Also, you might start with a single video card rather than the dual system and add the second card back once you have gotten the OS up and drivers in place.