I ordered my new Mac Pro "short loaded" with 16gb of ram. Expecting 1866 ram to come down in price over time, and even at today's prices it is cheaper to buy elsewhere. What I've noticed at Crucial and OWC sites is that they state you cannot mix different sized ram chips. "Installation of 16GB modules requires all previous lower density modules to be removed. You cannot mix Registered and Unbuffered memory in the same system." I find this statement confusing. I've mixed different sizes of ECC before on other computers. Is this a limitation of Intel's memory controller? An Apple imposed limitation for stability reasons, or just ram merchant snake oil?