The Gigabyte Z77-HD4 comes with one PCI Express 3.0 x16 slot, one PCI Express 2.0 x16 slot (working at x4), two PCI Express 2.0 x1 slots, and two PCI slots.
Usually, on motherboards based on the Z77 chipset, the first two PCI Express x16 slots are controlled by the CPU, and the other PCI Express x16 slots are controlled by the chipset, operating at a lower speed (almost always x4) and only compatible with the 2.0 specification, which offers half of the 3.0 bandwidth. On this motherboard, however, only the first PCI Express x16 slot is connected to the CPU, therefore always working at x16, using the 3.0 specification if you install an “Ivy Bridge” processor. The second PCI Express x16 slot is connected to the chipset, working at x4 and using the 2.0 specification. This configuration clearly shows that this motherboard is an entry-level model.
When installing dual-slot video cards, you “kill” the slot immediately to the left (looking at the motherboard with its rear connectors facing up) of the slot being used.
The PCI Express x16 slots support the CrossFireX technology. SLI is not supported.