What CPU are you getting? With an i5 750 you will not be able to set RAM to run faster than 1333 MHz without OCing your CPU. An i7 860 is limited to a top frequency of 1600 MHz without OCing your CPU. However the difference between the two frequencies is often marginal and not always noticable outside of benchmarks.
However, there can still be good reasons to get 1600 RAM. For one, at times you can find 1600 RAM that is cheaper than 1333 RAM with similar specs.
If you haven't read this yet, Tom's has an article on the P55 that does a comparison at different speeds and timings: http://www.tomshardware.com/reviews/core-i7-870-1156,2482.html
They also did a review of value RAM you may find helpful: http://www.tomshardware.com/reviews/ddr3-4gb-p55,2462.html