Often with AMD and other MOBOs, sometimes it's not an option; only "Support for DDR3 2000(OC)/1333/1066 MHz memory modules" is listed.
However, Memory Clock -> 8.0 is available so set it; I noted the setting in the manual, and the 955 can handle the 1600 MHz w/o messing with the CPU Frequency(MHz) settings.
I would also enter the CAS timings: 9-9-9-24-2N in the DRAM Configuration. ref http://www.gskill.com/products.php?index=222
You may be required to raise the DRAM Voltage Control 1.55v~1.60v.