The REASON the low-latency 1600MHz of DDR3 is the best value is that it is fast enough to avoid BOTTLENECKING games.
Put another way, you could buy FASTER memory but since the bottleneck is likely elsewhere (such as the CPU) it won't make much difference. You can easily find benchmarks for this. There are only a FEW games that benefit from greater than 1600MHz.
The (OC) question:
I believe saying (OC) after the RAM under the motherboard specs means that the CPU has to be overclocked for the RAM to achieve this.
*The lowest frequency, 1600MHz, which does NOT have (OC) after it. This apparently means that to utilize 2400MHz you would have to overclock your CPU by 2400/1600 or 50% which is very unlikely.
HOWEVER, in reality this doesn't appear to be the case. My Asus Z77 Sabertooth lists up to 1866MHz only however 2133MHz memory is working WITHOUT the CPU being overclocked.
- more than 1600MHz, low-latency generally a waste of money
- the (OC) is supposed to mean "achieve by overclocking the CPU" but in reality the numbers don't always jive (perhaps due to a BIOS update ?)