Yes, you can use a single AMD or Nvidia card.
With the GA-P55M-UD2 board, which I have in my system, running CF is possible, but not recommended. SLI is not possible. The board only runs one PICe x16 slot to the CPU. There is a second slot that can run at x4, however it runs through the PCH.
In other words, although CF is allowed it is not recommended because the two cards will run at x16, x4. For CF or SLI, you want a board that runs the two cards at x8, x8 (for socket 1156 boards). Also, a board must be licensed by, and the manufacturer paid a fee to, Nvidia for SLI to be allowed.