Gigabyte EX58 Extreme has three PCI Express 2.0 x16 slots, one PCI Express 2.0 x4 slot, one PCI Express 2.0 x1 slot and two PCI slots. The third PCI Express 2.0 x16 slot (the orange one) does not work at x16 mode, of course (since the Intel X58 chipset supports only two x16 lanes): it works at x1 when both the x16 PCI Express slots are working at x16 speed or at x8, if you decrease the speed of the second slot to x8. In this case the first slot will continue working at 16x. Using the manufacturer's language, you have two possible configurations: x16/x16/x1 or x16/x8/x8. If you have only one or two video cards, they run both at x16.
Note that this motherboard has one PCI Express x1 slot, but this slot cannot be used if you install the cooling solution that comes with the motherboard. But still, if you have a x1 card you can install it on the x4 slot (or on the third PCI Express x16 slot, if is it still available).

The motherboards with more than one PCI Express x16 slot based on Intel chipsets have always supported CrossFire configuration, but traditionally they do not support nVidia's SLI. Gigabyte EX58 Extreme, however, supports SLI with two (like ASUS P6T Deluxe) or even three video cards (which is not supported by P6T). You'll find on market motherboards based on Intel X58 chipset with SLI support and without it - as long as we know, the Intel "Smackover" does not support, for example. This support depends on whether the manufacturer paid license to nVidia and passed their certification process or not - which, by the way, increases the cost of manufacturing, which obviously reflects on the cost to the end user. Support for SLI is enabled at driver level, so no nVidia bridge chip was necessary on the motherboard.

 
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Figure 2: Slots.

This motherboard does not require extra power for the PCI Express slots, unlike Intel DX58SO.