What is the speed of ram should I use for intel core i7 3930K, how much mhz
The real question is which RAM modules the motherboard supports. The question can not be answered simply by the processor specifications. It's the motherboard that dictates which memory modules can be used.
Doesn't really matter, there's little to no real world difference between RAM speeds for your X79 system. Only difference you'll notice is in benchmarks and your wallet so just go for the 1600Mhz kits (which are usually the same price as 1333Mhz ones). Make sure you get 4 sticks so you can run them in quad channel and take full advantage of your system.
But I am curious as to what you need a 3930K for and what the rest of the build looks like.
I see that you've made several threads about this build you seem to be doing, how about you just crate a new thread with all your components listed, so we can get a better picture as to what you are doing.