The ASRock 990FX Extreme9 comes with four PCI Express 2.0 x16 slots, one PCI Express 2.0 x1 slot, and one PCI slot.
The configuration of the PCI Express x16 slots is shown in the table below. In summary, the first slot always works at x16 speed; the second slot always works at x4 speed. The speed of the PCIE4 slot will depend upon whether or not you have a video card installed in the PCIE5 slot. Therefore, when installing two video cards, install them on the first (PCIE1) and third (PCIE4) slots for best performance.
If you plan to install a dual-slot video card in the fourth PCI Express x16 slot (PCIE5), you will have to buy a case with at least eight slots. (Computer cases usually have seven.)
When installing dual-slot video cards, you “kill” the slot immediately to the left of the slot being used (looking at the motherboard with its rear connectors facing up). This means that you can’t install a dual-slot video card in the second (PCIE3) PCI Express x16 slot, because otherwise you will block the third (PCIE4) slot, which is faster than the second. Therefore, even though this motherboard has four PCI Express x16 slots, only three of them can be used with dual-slot video cards.
The PCI Express x16 slots support both SLI and CrossFireX technologies.