First you need a CPU that's capable of running 2133MHz RAM, not every 2500K or 2600K could do that without raising VCCIO voltage.
I run my Kingston HyperX Blu 1600MHz CL9 @ 2133MHz with standard 1.65V and 10-11-10-30 timings on my P8Z68-V Pro/Gen 3.
I didn't have to raise VCCIO voltage (memory controler voltage), so I left it on default 1.087V.
If you have Ivy Bridge CPU then it's almost guaranteed that you'll be able to run 21333MHz or higher because 3570K and 3770K do have better memory controller than 2500K/2600K..
You just set your RAM speed to 2133MHz, set your RAM voltage and timings to recommended specs of your memory kit.
Thanks for the comment. I'm mostly 95+ percent stable with my 2600K. I just want to be absolutely sure that I am as just once some games launcher crashed before I could play the game so I wonder. This only happened once. Seems stable. But I should be able to raise that one notch just to be safe?