Go to the overclocking menu. Select "Manual" Under "DRAM Timing"
Select DRAM Timing and there is an option to adjust memory speed.
I have 2 x 4gig sticks of corsair vengeance 1600, and here is the problem I have found:
The TA990FXE will only properly support 1333mhz, if you select 1600mhz the board will only have 4gigs of "usable" memory.
Unless this issue is specific to just me, then you want to run 1333 until BioStar releases a bios update to fully support 1600mhz of corsair memory.
Should be noted, that I don't think the Corsair memory is on the QVL memory support list yet.