i7 6800k and Ram

Solution
Use the motherboard as a guide for that.

However, it's not much of an issue since memory have several profiles and can default to lower ones as needed. I usually just choose "XMP" and if the memory profile exists already in the BIOS it should choose the best profile.

Example kit: http://pcpartpicker.com/product/p6RFf7/corsair-memory-cmk16gx4m2b3200c16

There's a diminishing return above 2666MHz CL16 but it does exist, especially with the 6C/12T i7-6800K when heavily stressed (the more data can be processed per unit of time, the higher the memory bandwidth needs to be).

I'm not sure you can easily find much real-world benefit above 3200MHz. (dual channel if not clear, as a single stick gives half the bandwidth)
Use the motherboard as a guide for that.

However, it's not much of an issue since memory have several profiles and can default to lower ones as needed. I usually just choose "XMP" and if the memory profile exists already in the BIOS it should choose the best profile.

Example kit: http://pcpartpicker.com/product/p6RFf7/corsair-memory-cmk16gx4m2b3200c16

There's a diminishing return above 2666MHz CL16 but it does exist, especially with the 6C/12T i7-6800K when heavily stressed (the more data can be processed per unit of time, the higher the memory bandwidth needs to be).

I'm not sure you can easily find much real-world benefit above 3200MHz. (dual channel if not clear, as a single stick gives half the bandwidth)
 
Solution