With DDR3 RAM the frequency and latencies make little difference unless you can run the RAM at about double the default CPU/RAM frequency.
Your best performance value will be one 8 GB. (2x 4 GB.), 1600 Mhz. matched, tested RAM kit, NOT multiple DIMMS as they may not function together well or at all.
If you're flush with cash and feel lucky you can buy expensive 2200 Mhz. RAM and try to OC it to 2000+ Mhz. for a few more FPS in games, but it's hardly worth the price.