This isn't really specific to Linux but, hopefully, somebody on here will know the issue and have some ideas.
I have a GA-990XA-UD3 (rev 1.0 as far as I know) and an FX8370 overclocked to 4.5GHz and 4 Corsair 8GB RAM sticks. The issue is that if I try to change the RAM multiplier to 8.0 or above to get over the 1333MHz clock speed the system reports and error and resets it to 1333. I also tried changing the voltage to 1.6V in case that mattered but the same thing happened. I don't, unfortunately, have the RAM part numbers to dig out other settings but I understand that some timings may need changing also?
Has anybody got 4 sticks of RAM in one of these motherboards clocked at over 1333MHz and, if so, how?

OK, so now I'm embarrassed. I had googled my issue and found some RAM timings that looked right, applied them and got nowhere. I think I need to educate myself about the various RAM timings because when I finally remembered I bought my RAM form Amazon and searched my orders I found I ordered CML32GX3M4A1600C10B and googling that sent me to the Corsair page for it and the specification "Tested Latency: 10-10-10-27" which, when entered into my BIOS in the most likely looking places (i.e. "educated guess") now has my RAM set at 1600MHz -- which is the rated speed of the sticks.
Now I may look into overclocking but I mark this as solved since I'm now, finally, getting what I paid for.

Good Job and well done.

I do feel embarrassed and also, after looking into latency a little, a bit disappointed that the settings are so high. It almost looks as though DDR is being mis-sold as, for example, 1600 when it's only actually addressable at 1/10th of that speed or less. I'll certainly be more vigilant next time I build a system -- tweaking this one has taught me a few lessons in details.