There are two steps in you BIOS you need to pay attention too. First is the MIT ( Motherboard Intelligent Tweeker ) section, you need to change the "Set Memory Clock" to manual. Then the option below it, memory clock, will light up and you can change the basic setting from 1333 to 1600. The advanced section, DRAM E.O.C.P can be used if you want to set the CAS latency and the rest of the more advanced settings. Check with you RAM manufacturer to get the rest of the settings. The first 4 starting with CAS latency are the most important. That is why you typically see memory timings of 1600, 9-10-9-27 or something similar!
You really should consider reading the manual:
http://download.gigabyte.us/FileList/Manual/mb_manual_ga-970a-d3_e.pdf