I could link articles, but that would waste my time so I'll give you some overclocking in a nutshell. If you do not have an unlocked multiplier (Intel K series, AMD BE series), you typically must overclock your base clock that your CPU and RAM both run off of.
Because of this, it is a lot easier to overclock with RAM that can handle faster speeds. There are several settings that will still allow you to use lower RAM speeds, but only to a point. 1333 should work. You would likely run into problems with 1066. I prefer 1600 or faster.
As far as benefiting your processor best--the faster your RAM runs, the better--this includes latency. So 1333MHz DDR3 with Cas7 will do about the same as 1600MHz Cas9. So basically don't just get 1333MHz Cas9 when 1600MHz Cas9 is the same price. The only way to find out exactly what's best is to run benchmarks on your system after setting up RAM timings in your BIOS.
If you want to just put the parts together, no tweaking BIOS settings or future overclocking down the line, then 1333MHz is for you. 1600MHz stuff works great, but you'll have to manually set it to manufacturer specs.
But I'd get 2000MHz for myself (probably bad advice).