Spec me some memory (or knowledge)

I am hoping for some specific advice.

I am trying to build myself a watercooled rig, and I don't know really what I am doing besides research and more research. Right now I am feeling pritty lazy and hope someone else can help me.

I was considering the Asus Z170 delux motherboard. I want the Skylake 6700k CPU, and probably the nvidia GTX1080. So far so good. Where i am currently stuck is memory, and not knowing what I don't know. I looked at the motherboard manual and see

4 x DIMM, max. 64GB, DDR4 3400(O.C.)* / 3333(O.C.)* / 3200(O.C.)* / 3100(O.C.)* / 3000(O.C.)* / 2933(O.C.)* / 2800(O.C.)* / 2666(O.C.)* / 2600(O.C.)* / 2400(O.C.)* / 2133 MHz, non-ECC, un-buffered memory
Dual channel memory architecture
Supports Intel® Extreme Memory Profile (XMP)


I'm lost. What does the O.C. mean? What's a good recommend for the system based on the gear I already have. What's the middle ground (price sweet point) and what's the best this mobo can handle.

If anyone has a suggestion/tips, Thankyou.
[/lazy]

Over Clocked. Most motherboard class anything over 2400Mhz as overclocked.

The best speed for the memory depends on how far you overclock the CPU. 3000mhz should cover all but the fastest 6700k's.

Choice made then I believe

Corsair Vengeance LED 16GB (2x8GB) DDR4 PC4-24000C15 3000MHz Dual Channel Kit

Thank you, have a pint!