I'm not 100% sure on this but i thought that quad sli was done by buying graphics card with two seperate GPU's on themlike the 7950 GX2 cards.... then you simply put one of them in each PCI-e x16 slot, hence 4 GPU's...
Thats what i always figured anyway... if i am right then any SLi motherboard should work, the 680i should be MORE than capable of doing that
dragon