The ASRock X79 Extreme6/GB comes with three PCI Express 3.0 x16 slots, one PCI Express 2.0 x1 slot, and two standard PCI slots. Socket LGA2011 processors have a total of 40 PCI Express 3.0 lanes for video cards. This allows a very high-end configuration for the PCI Express x16 slots, but the exact configuration used will depend on the motherboard model.
The first and second PCI Express x16 slots always run at x16 speed, while the third one always runs at x8 speed.
When installing dual-slot video cards in the first and second PCI Express x16 slots, you “kill” the slot immediately to the left (looking at the motherboard with its rear connectors facing up) of the slot being used. You can install up to two dual-slot video cards using a regular seven-slot computer case, but the installation of a third dual-slot video card requires you to have a case with at least eight expansion slots.
All PCI Express x16 slots support both SLI and CrossFireX technologies.