The ASUS P8Z68-V PRO comes with three PCI Express x16 slots, two PCI Express x1 slots, and two standard PCI slots.
The first two PCI Express x16 slots are connected directly to the CPU integrated PCI Express controller. When only one video card is installed, the first slot works at x16 speed, but when you install two video cards, each slot works at x8 speed. These slots support both SLI and CrossFireX technologies. The third PCI Express x16 slot may work at x4 or x1 speed. In order for it to work with x4 speed, you need to disable the two x1 PCI Express slots, the two USB 3.0 ports labeled as USB3_34 (which are the ones available as a motherboard header), and the eSATA-300 port in the motherboard setup. Otherwise, this slot will work at x1 speed.
If you install a dual-slot video card in the first PCI Express x16 slot, you will “kill” one of the PCI Express x1 slots. If you install a dual-slot video card in the second PCI Express x16 slot, you will “kill” one of the standard PCI slots; and to install a dual-slot video card in the third PCI Express x16 slot, you will need a case with at least eight expansion slots (cases usually have seven expansion slots), and you may block the buttons and headers located at the motherboard edge.
It is important to understand that Intel chipsets no longer support standard PCI slots, and the PCI slots are provided by an ASMedia ASM1083 bridge chip.