The default ram setting on many x58 boards is 1066. Try the manual setting and see what ram speeds are listed. 1333 or 1600 is plenty fast if you don't want to overclock. If your ram works fine at the lower speeds, I would leave it alone. You want to run the ram in sets of threes, and 6 gb is plenty. I wouldn't mix ram or try using 2 sticks instead of three.