One of the main new features from socket LGA1156 processors is the presence of an integrated PCI Express 2.0 controller inside the CPU. This controller supports one x16 connection or two x8 connections. Usually on P55-based motherboards the manufacturer puts two PCI Express 2.0 x16 slots on the board, with the first slot working at x16 when only one video card is used or both slots working at x8 when two video cards are used.
ASUS P7P55D PRO, however, has three PCI Express 2.0 x16 slots, each one with a different color. The first two (blue and light gray) are connected to the controller inside the CPU and thus the blue slot works at x16 only one video card is installed, but when two or three video cards are installed both the blue and the light gray slot work at x8. The third slot, which is dark gray, is controlled by the chipset and works at x4.
ASUS P7P55D PRO also has two x1 PCI Express slots and two standard PCI slots.
This motherboard from ASUS supports both CrossFire and SLI configurations. Keep in mind that SLI support on P55-based motherboards will depend on whether the manufacturer licensed this technology from NVIDIA or not, i.e., not all P55-based motherboards have SLI support. On the ASUS line-up only P7P55D LE and the standard P7P55D do not support SLI.