Between 1333 and 1600, I would say there is a difference, depending on your timings on the RAM, too. Between 1600 and 1866, I don't think many boards or processors use the extra speed all that well. Getting 1600 RAM with good timings is the best bet, in my opinion. (Better timings = lower. 8-8-8-24 is better than 9-10-9-27)