Extreme chips actually have an X in their model numbers, usually about $1000 as well. X in X79 doesn't stand for Extreme, just a type designation, like Z, P, and B.
There are many CPUs on the X79 platform that are fine for gaming, though you do pay a bit of a premium on the motherboard. But you get quad channel memory and 40 PCIe lanes rather then 32.
i7-4820k, I7-3820 are basically equivalent to the Haswell and Ivy 3770k, and 4770k.
3930k, 4930k are hex core CPUs, really not necessary for gaming.