The ASRock Z77 Extreme4-M comes with two PCI Express 3.0/2.0 x16 slots, one PCI Express 2.0 x16 slot (working at x4), and one PCI Express 2.0 x1 slot.
The two PCI Express 3.0/2.0 x16 slots are controlled by the CPU, with the first slot working at x16 when only one video card is installed, and with both working at x8 when two video cards are installed. Since these slots are controlled by the CPU, they will offer PCI Express 2.0 bandwidth (8 GB/s at x16) when a “Sandy Bridge” CPU is installed, but PCI Express 3.0 bandwidth (16 GB/s at x16) when an “Ivy Bridge” CPU is used.
The third PCI Express x16 slot (PCIE4) is controlled by the chipset, always works at x4 speed, and uses 2.0 bandwidth. You won’t be able to use this slot if you install a dual-slot video card in the second PCI Express 3.0/2.0 x16 slot.
The PCI Express x16 slots support both SLI and CrossFireX technologies.