In reference to memory the clock speed is the speed of transferring the information, or in other words it is like a bus speed. Latency timings is turn around time. So when memory gets a piece of info, according to your latency timings, that is how many cycles the memory holds it before transferring it. You basically want something in the middle. From what i've heard there isn't much of a difference after 1600 mhz, but timings can still have an impact. Thats about all i know when it comes to memory
1600mhz DDR3 with decent timing is going to be the most benefit. When you go much further, there really isn't much if any performance increase. The tighter the timing on the RAM, like was said by underdog, the better it performs.
So just find a set of good timing DDR3 1600mhz and you'll be happy. Overclocking it from that point really just isn't worth the time, effort, or performance gain.
Also, as you may notice, the faster speed ram, the looser the timings. That basically goes the same when overclocking the ram. A lot of times when overclocking ram, stability issues can actually be solved simply by loosening the timings. So if you're wanting even MORE gains from ram, get the tightest timing ram you can find and that your board will support. If that isn't enough, try tightening the timings, you might actually be able to get better timings than the ram is defaulted to.