The ASUS SABERTOOTH Z77 comes with two PCI Express 3.0/2.0 x16 slots, one PCI Express 2.0 x16 slot, 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.
The third PCI Express x16 slot is 2.0 and always works at x4 speed. If you want to install a dual-slot video card in this slot, you will need a computer case with at least eight expansion slots. (Computer cases usually have seven.)
The PCI Express x16 slots support both SLI and CrossFireX technologies.