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. MSI P55-GD80 uses this scheme, plus it has a third PCI Express x16 slot working at x4 and being controlled by the chipset. MSI P55-GD80 also has two x1 PCI Express slots and two standard PCI slots.
This motherboard from MSI 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 MSI line-up P55-CD53 and P55-CD45 do not support SLI.