The 1333 thing just means that anything faster will default to 1333 when first installed, but you'll just need to either set the right settings manually in the BIOS or enable the X.M.P. profile (recommended). That's "technically" overclocking, but doesn't affect the CPU, and you're just setting the RAM to what it's supposed to be anyway.
It'll work perfectly fine and flawlessly with faster than 1333 RAM, though (take note of my 2500K running brilliantly with 1600 RAM).
My recommendation is to go with 1600 RAM right off the bat. No harm in it at all.