That 1333 isn't really mhz clock speed. That will depend on many factors including your CPU clock speed. As well, DDR RAM is supposed to transfer twice the data of non DDR at the same clock speed...or something like that. Here's more on the numbers game. The RAM cited here is a bit dated but you can get a better idea of what the numbers might mean. As long as you use identical RAM in all 6 slots, you shouldn't experience performance drops.

http://en.wikipedia.org/wiki/DDR_SDRAM