The ASUS P8P67 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 (blue) works at x16 speed, but when you install two video cards, the first two slots work at x8 speed. These slots support both SLI and CrossFireX technologies.
The third PCI Express x16 slot (black) works at x1 or x4 speeds. Since the P67 chipset doesn’t provide enough PCI Express lanes, by default the third PCI Express x16 slot works at only x1 speed. If you want to make it work at x4 speed, you will have to manually disable the eSATA ports, the internal USB 3.0 header (USB3_34) and the PCI Express x1 slots on the motherboard setup. If this motherboard had a PCI Express switch chip, it would be able to automatically switch the speed of the third PCI Express slot.
It is always good to remember that PCI Express x16 slots accept x1, x4, x8, and x16 expansion cards directly.
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 (computer cases usually have seven), and you may block the USB and FireWire 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 ASM1085 bridge chip.