PCI x16 is the full size pci-e slot, there are both electrical and physical specs. Dual x16 means that if you have x16 (full length) cards in the two slots they will both run at x 16 speed. x16/x8/x8 means that if you have 3 x16 cards in these slots, one (probably the first one)( will run at x16 and the other two at x8 speed. quad x8 means that if you have 4 x16 cards in the slots they will all run at x8 speeds. Although they are all x16 size they wont necessarily run at x16 speed. If you are going to run quad sli you are probably better off going with an x series chipset (x79 or x99) as they offer more pci-e lanes.