With DDR3, you'll see three main categories. 1066, 1333, and 1600, though there are higher. That number refers to the clock speed of the RAM...how many megahertz it runs at. Technically the highest official standard is 1333, so anything higher has to be "overclocked" to its rated speed.
1600 is what I'd get.
Of course, there is more to it than that. The timings come into play as well as the voltage. Lower voltage is good. You'll see something like "7-7-7-24" or "9-9-9-25" on its specs page...that's the timing. Lower numbers are generally good.
Some motherboards, particularly intel's x58 boards, are picky about what RAM they use. That's probably what you'll want, so be aware of that.