A larger motherboard means more room for expansion. The P7P55D Deluxe sports three PCI Express x16 slots, two x1 slots, and two PCI slots.
Broken down, though, the true link speeds are a bit different. The first x16 slot runs at x16 signaling with a single card plugged in. It drops to x8 with two graphics cards installed in an SLI or CrossFire arrangement, due to Core i5's on-die capabilities. The third slot runs at x4 speeds so that there's enough PCI Express left over for the two x1 slots, one of the two gigabit controllers, and additional SATA connectivity.
You can also see the unique retention mechanism attached to all three x16 slots, which make it easy to secure and release graphics cards from either side of the slot. This is a big improvement on some of the other solutions out there that require pressing on the side of the slot or pulling a tab away from the slot to free an installed add-in board.