The CMX12GX3M3A2000C9 aren't my favorites nor do I really recommend any XMS3 set.
Instead for 3x4GB, DDR3-1600 CMP12GX3M3A1600C9 or DDR3-2000 CMT12GX3M3A2000C9 which I run on my 980X with a minimum BCLK of 160MHz+.
edit: the DDR3-2000 are ONLY, O-N-L-Y, if you plan to significantly OC you 990X e.g. 4.2GHz or higher otherwise without the BCLK as mentioned I can almost guarantee problems and instability. I run 4.2GHz~5.0GHz on water; BCLK of 200MHz works out great but e.g. 180MHz you're either running them slow or high in comparison to Rated. So plan ahead!