If you have the option 1866Mhz is the optimal speed for gaming. Otherwise, 1600Mhz is fine. Go with the set that is either lower in price or lower CAS latency (7 or 9 is faster than 10 or 11). Keep in mind that speed like 1600 or 1866 doesn't really effect price. It's the line that the DRAM is, the CAS latency, the number of GB on each modules, and the number of modules. Also consider which warranty you prefer more.