One of the main features of socket LGA1156 processors is the presence of an integrated PCI Express 2.0 controller inside the CPU. This controller supports one x16 connection or two x8 connections. The Intel P55 chipset provides a total of eight x1 PCI Express lanes.
Due to its reduced size, the Gigabyte H55N-USB3 has only one expansion slot, a PCI Express x16. Keep in mind that this slot accepts x1, x4 and x8 cards.
Of course the H55N-USB3 can’t support CrossFireX or SLI configurations.