I am thinking of ordering the Area-51, with the i7 980x extreme processor.

I will order only the base RAM (6GB 1333MHz), which I plan to upgrade.

I'd like to install 12GB of the trichannel RAM.

I think 12 GB (3 x 4GB) 1600MHz would work;

           but would 12 GB (3 x 4GB) 1866MHz work ?

           - what about 12 GB (3 x 4GB) 2000MHz ?

I don't know much about this stuff, but I've read that high frequency RAM may stress the CPU, actually making the computer run slower.