One of the main features of the socket LGA1156 CPUs is the presence of a PCI Express 2.0 controller integrated into the processor. This controller supports a x16 connection or two x8 connections. P55-UD6 seems to have three PCI Express x16 slots, but looking carefully we see only the first one is really x16. The second slot works at x8 speed, and is good to remember that, if you install a video card on it, the first slot works also at x8. Third slot is actually a PCI Express x4 slot controlled by the chipset. You can install a regular video card in this slot, but it will work at x4 speed. Besides that, this third slot does not offer support for SLI or CrossFireX arrays. Remember the SLI support on P55 motherboards will depend if the manufacturer has licensed this technology with NVIDIA and, so, not all motherboards based on Intel P55 chipset support SLI.
Gigabyte P55-UD6 also offers two PCI Express x1 slots and two standard PCI slots.