If you're referring to CPU overclock many are not reaching their overclock hopes from the 4790K simply because with a 4ghz base clock, Intel is already giving you a great starting point which unfortunately means your top end is limited to the cooling you're running.
The 4790K overclocking study linked below will let you get an idea of what overclock levels they reached with the cooling they used so you can understand my first statement.
If you are referring to the Memory frequency overclock alone then since your top end CPU overclock is cooler limited, I'd just go ahead for the 2400mhz G-Skill Trident modules out of the gate.
What will you be doing with the rig? Primarily gaming I'd go 1866/8 - if doing multi-tasking, video, Imaging, CAD, VMs - anything like that I'd look to 2133/9 or 2400/10 - the GSkill Tridents are my favorites for the 1150 socket
If you're referring to CPU overclock many are not reaching their overclock hopes from the 4790K simply because with a 4ghz base clock, Intel is already giving you a great starting point which unfortunately means your top end is limited to the cooling you're running.
The 4790K overclocking study linked below will let you get an idea of what overclock levels they reached with the cooling they used so you can understand my first statement.
If you are referring to the Memory frequency overclock alone then since your top end CPU overclock is cooler limited, I'd just go ahead for the 2400mhz G-Skill Trident modules out of the gate.
I am not discounting what TM1 has already told you, because what you actually do with your computer not only affects your speed choice but how much memory you choose to run as well, I'm just adding my 2 cents!