For a 4790k and a Z97 board of that caliber, I would never, unlike many older recommendations, run a 1333 or 1600mhz module. 1866 to 2133mhz would be the range I'd want to install, and probably, for high end applications, 8-16GB depending on if I was running serious enough loads to warrant 16GB.
I always recommend using, according to what you can afford, the fastest supported module with the lowest latency and the highest amount you can install that makes any sense according to your probable usage. If you will be running high end video or audio applications, simultaneously streaming ANYTHING, or using several high end apps simultaneously, two 8GB modules would be recommended.
You should be able to easily run two 8GB 1866 or 2133mhz modules on that board with that cpu. The fact that the boards specs say "OC" is misleading. It simply means you need to enable the modules XMP profile in the bios before it will run at speeds beyond 1600mhz. The gaming 5 actually supports all the way up to 3300mhz modules as does the CPU. Not much to be gained beyond 2400mhz though.