I just bought a new computer setup:
Patriot Viper III 2X4GB kit
Intel Core i7 3770K+Stock cooler (for the time being)
and an AMD Sapphire HD7970 Vapoer-X 3GB GHZ.
I tried to install the GPU on the X16 slot with no success, tried a different card (Nvidia Geforce 9500GT 1GB) and it worked just fine, and only when I connect the GPU to the second PCIE slot (operating at x8) it worked.
I tried just about everything! including messing with every possible option in the BIOS, updating it to the latest version and a full CMOS reset. I also checked the GPU in another motherboard and it worked.