I'll also be getting an i5-4670k and will be using it for gaming. Is 8GB DDR3 1600 good or should I get a higher speed? Would G.SKILL Ripjaws X Series 16GB (2 x 8GB) 240-Pin DDR3 SDRAM DDR3 1600 (PC3 12800) work?
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Well if it's main use is gaming then 1600 is the sweet spot. Anything above that will be a complete waste of money, as it won't improve your gaming experience nor will it speed anything up, you really won't be able to tell any difference.
Also 8GB is fine as well, you would only need 16GB or more, or faster RAM if you was video editing, large multi layered Photoshop projects, rendering .e.t.c....
If you would like, I can link you to a video where a tech team test 4GB vs 8GB vs 16GB for gaming??? It proves my point that even in the most demanding of gaming, only around 6/7GB was being used even with 16GB installed.
This is the list of supported RAM for that MoBo so you can see exactly what RAM is compatible with that board: ASRock Z87 X4