The Biostar Hi-Fi Z77X comes with two PCI Express 3.0/2.0 x16 slots, one PCI Express 2.0 x16 slot (working at x4), and three PCI Express 2.0 x1 slots.
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.
If a third dual-slot video card is installed (in the third PCI Express x16 slot), you will need a case with at least eight expansion slots.
According to Biostar, only the PCI Express x16 slots controlled by the CPU support SLI and CrossFireX technologies.