AMD FX CPU architecture supports the use of 1866MHz MHz speed with one DIMM per channel and 1600MHz speed with two DIMMs per channel. Higher frequencies require overclocking the system.
Leaving BIOS memory settings on default will probably result in conservative SPD (Serial Presence Detect) timing parameters. You can manually choose memory clock settings, selectively modify memory timings or use XMP settings. The eXtreme Memory Profile (XMP) allows you to overclock compatible DDR3 memory to perform beyond standard specifications.