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The MSI 990FXA-GD80 comes with four PCI Express x16 slots, two PCI Express x1 slots, and one standard PCI slot.
The first PCI Express x16 slot (PCI_E2) always runs at x16 speed; the second PCI Express x16 slot (PCI_E4) always runs at x8 speed; the third PCI Express x16 slot (PCI_E5) runs at x16 only when the second PCI Express x16 slot (PCI_E4) is empty (otherwise, it runs at x8 speed); and the fourth PCI Express x16 slot (PCI_E6) always runs at x4 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.
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” the standard PCI slot. If you install a dual-slot video card in the third PCI Express x16 slot, you will “kill” the fourth PCI Express x16 slot. And to install a dual-slot in the fourth PCI Express x16 slot, you will need a case with at least eight expansion slots (cases usually have seven), and the video card may block the buttons and headers located at the edge of the motherboard.
All PCI Express x16 slots support both CrossFireX and SLI modes.
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