look for dual chanel memory with the lowest clock latency cl at 1600.
the best will be 2x4= 8gb of 1600 mhz with a clock latency of 9 the lower the clock latency the faster the memory.
if you look at a 1866 mhz you will se it has a higher clock latency like 11-12.
the preformance benefit will only be 2%.
how much memory do you want and what motherboard do you have?