I had alot of troubble running at 2000mhz on this board....after running 1600mhz for a while i was only getting 4g of my 6g of ram being reconised by windows.... allthough all 3 sticks pass a super pi and memtest at 2000mhz by them selves 1 at a time....as soon as i put the g skill stick int he 4th ram slot it won't be reconised.....but if i use a ocz stick in the 4th slot all is fine.....ram was rma'd still have the same issue.... it cant be the ram because they all work fine 1 at a time..... although its weird for a ram slot 2 be selective on what works and what doesnt.....IS THIS A MOTHERBOARD ISSUE??
More aboutslot issue please
what is your OS?
Have you tried the ram in different combos two x two???:)
Does the BIOS recognized all 6GB at the correct speed? If the BIOS sees all the RAM at the correct speed, then it might be an OS issue. Don't forget to enable memory mapping in the BIOS as well.