Any DDR3 RAM rated at those speeds. 1600 is the sweet spot right now. Sometimes mobo manufacturers even list specific RAM models they tested to make sure they work. Those are good choices too.
It's mostly a cost thing. You probably won't notice the performance difference in 1600 vs 1866. But you'll notice the cost difference to get the higher end RAM.
I'm still confused as to what the numbers even indicate and how that all relates to making an optimized system but after two hours of research and still not getting it I give up. Thanks calinkula going with 1600.
It relates to the speed of the RAM. Much like the frequency of a CPU, but it's a little different because of timings. Timings is how many clock cycles it takes for the RAM to respond to a request (lower is better).
Anyways, to keep it simple just go with some DDR3 1600 from a reputable brand.