Intel X58 chipset has 36 PCI Express x1 lanes and on Gigabyte X58A-UD7 the manufacturer arranged these lanes in four PCI Express x16 slots.
The first PCI Express x16 slot works at x16 if the second slot is empty. If there is a video card on the second slot, both the first and the second slot will work at x8. The third and fourth PCI Express x16 slots work the same way: the third one works at x16 or x8, depending on whether the fourth slot is empty or not. So if you have two video cards it is really important to use the first and the third (and not the second) PCI Express x16 slots in order to get the best performance possible. In other words, this motherboard supports the following configurations: x16; x16/x16; x16/x8/x8; or x8/x8/x8/x8. The slots are compatible with SLI and CrossFireX arrays.
The motherboard also comes with two PCI Express x1 slots and one standard PCI slot.
The fourth PCI Express x16 slot is located at the edge of the motherboard, so if you install a dual-slot video card you will need a case with eight or more expansion slots. Also the first three slots are close to each other, so if you install a dual-slot video card on the first or second slots you will lose the slot right next to it.