The ECS X79R-AX comes with four PCI Express 3.0 x16 slots and two PCI Express 2.0 x1 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 third PCI Express x16 slots (gray) always run at x16 speed, while the second and fourth PCI Express slots (white) always work at x8 speed. ECS hit the bull’s eye by using different colors to identify the speed of each slot.
If you are installing two video cards, make sure you install the second one in the third (and not in the second) PCI Express x16 slot for the best performance.
When installing dual-slot video cards in the second and third PCI Express x16 slots, you “kill” the PCI Express x1 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 three dual-slot video cards using a regular seven-slot computer case, but the installation of a fourth 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.