Some chipsets and mobo implementations limit the total amount and speed of the RAM. For example, in a dual channel system, you can run 2-dimms @ 1600HMz but only run 4 dimms @ 1333MHz.
Haven't heard any such limit on the X-58 chipset but it would be best to check the mobo makers website and/or read the mobo manual to see if there are any such limitations or RAM disclaimer.