The thing to remember is that mobo is set up to run at its best efficiency as dual channel, which means 2 sticks of RAM.
http://www.gigabyte.com/products/product-page.aspx?pid=5174#sp
2 x DDR3 DIMM sockets supporting up to 16 GB of system memory * Due to a Windows 32-bit operating system limitation, when more than 4 GB of physical memory is installed, the actual memory size displayed will be less than the size of the physical memory installed. Dual channel memory architecture Support for DDR3 1600/1333 MHz memory modules Support for non-ECC memory modules Support for Extreme Memory Profile (XMP) memory modules