The ECS A990FXM-A comes with three PCI Express x16 slots, two PCI Express x1 slots, and one standard PCI slot. According to the manufacturer, the three PCI Express x16 slots really work at x16 speed. They allow you to install up to three dual-slot video cards using a regular seven-slot case. However, if you install a dual-slot video card, you will “kill” the PCI Express x1 slot or the regular PCI slot right next to it.
All PCI Express x16 slots support both CrossFireX and SLI modes.