The Gigabyte 990FXA-UD5 comes with five PCI Express x16 slots, one PCI Express x1 slot, and one standard PCI slot.
The first PCI Express x16 slot (PCIEX16_1) always runs at x16 speed; the second PCI Express x16 slot (PCIEX4_1) always runs at x4 speed; the third PCI Express x16 slot (PCIEX16_2) always runs at x16 speed; the fourth PCI Express x16 slot (PCIEX4_2) always runs at x4 speed; and the fifth PCI Express x16 slot (PCIEX8) always run at x8 speed. This way, in order to achieve the best performance if you are installing two video cards, you should install them in the first and third slots, not in the first and second slots as it would appear more logical.
There is no space between the PCI Express slots, so if you install a dual-slot video card in any PCI Express x16 slot, you will “kill” the slot right next to it.
All PCI Express x16 slots support both CrossFireX and SLI modes.