The north bridge of the Intel X58 chipset provides 36 PCI Express x1 lanes, while the south bridge (ICH10R) provides six more lanes.
The first two PCI Express x16 slots (PCIEX16_1 and PCIEX16_2) always work at x16 speed, and support both CrossFireX and SLI modes (the X58-USB3 comes with one SLI bridge). The third PCI Express x16 slot (PCIEX4_1) always work at x4 speed.
The X58-USB3 also comes with three PCI Express x1 slots and one standard PCI slot. Luckily two of the PCI Express x1 slots are placed to the right of the first PCI Express x16 slot, so if you install a dual-slot video card in it, the video card will block only one PCI Express x1 slot, still leaving two PCI Express x1 slots usable.
You will need a case with at least eight slots if you decide to install a dual-slot video card in the third PCI Express x16 slot (cases usually have seven slots).