Ya other's have nailed it. The middle slot is used for a physics card. The other use it holds if your going to run more than 2 displays. When you put 2 cards in the PCI-E X16 slots and run SLI you lose your 2 DVI outputs on the second card. So the only way to run 2 more displays would be put a third card in the 8X expansion slot and use as it as a 3rd and 4th monitor display. It would do nothing for your actual gaming just enable you to use more monitors.
