-Have you connected the auxilary power connectors of the graphic cards?
-If you did, try booting up with one card. If it doesn't work, try the other card.
-If you manage to boot up with one card the first time, try to get a good load on your system and monitor voltages. You've got a very good PSU but maybe it's just not good for all your hardware. They sometimes have cold-start trouble.
this motherboard is picky about the PSU. try with a real ATX2.0, you know, a PSU that doesnt need the adaptor for the power connector. I heard that, even if they are supposed to run with "old" ATX PSU, some have problems with it.
-Always put the blame on you first, then on the hardware !!!