Just a question I've been wondering about for these past couple of days. If I had some Corsair XMS3 1600MHz 2x2GB sticks in A1 and B1 and Corsair Vengeance 2x4GB 1866MHz in slots A2 and B2... would that make effect the other RAM? I remember hearing something to the effect that if you had more powerful RAM in the dual memory slots that that RAM would be the RAM that the computer reads, not the lesser power. Maybe that would only be in effect providing that A1 and 2000MHz and B1 had 1333MHz? Just curious if it would matter.
They will revert to the least common denominator in terms of speed (1600 in your case), but only if your BIOS is configured to run at 1600. To ensure compatibility, in this example, I would recommend you run at the stock 1333 speed (just set your BIOS to AUTO or default in terms of the memory settings). Your system would be more stable and you would see little appreciable performance difference.