I have an Asrock FATAL1TY 990FX Killer and it has 2 PCI-E x16's, 2 x1's and one x4 (in an x16). So, I understand that GPU's go in x16 slots right? And the second one for SLI goes in the x4 slot? Then why are there 2 x16 slots? And can I put x1's in x16?