If it's a gaming PC wouldn't you be mostly gaming in Windows 8.1 or 10?
Compare the available memory sizes and speeds in both QVLs and their prices for max mem. For instance, if you look at their latest memory list they list 32GB as 4x8Gs but the mobo only has two slots. And like other mobos, F4-2400C14Q2-128GRK is listed but since that is a 128GB kit it comes with 8x16G modues and you only need 2. And just like the 2400 speed G.Skill the CMD128GX4MM8A2666C15 is a single 128GB kit of 8x16, eight 16GB modules, of which you can't use 6.
No, you're going to have to vist the major manufacturer sites, Corsair, Crucial, G.Skill, Kingston, et. al., and using their Mem Finder app see what is available in memory sizes & timings, and then compare prices on Newegg and Amazon.
The board itself lists 32GB as max but if you can only install 16GB (2x8G) why bother? Why not buy a B150 or an H or Q 170, instead? You would have to see what mem is avaialbale on those platforms and mobos, too.
Why not look at a 4 mem slot micro ATX instead? Yes, maybe 16GB is fine now, but can you remember when everyone said the same thing about 4GB years ago? I suggest you build it for the future and install the max memory as the board can take. I think that buying high speed memory is a losing proposition and you would be better served installing a faster CPU. (If you read some of the comments on the Corsair forums many of their modules have problems running at the "rated" speed on the package, and many problems are attributed to them as causing problems on ASUS boards.)
On the ASUS Z170i the m.2 slot is on the back of the mobo, as far as I can see, so if you ever need to change it you will probably need to remove the whole board. That's very inconvenient, imo. Is there a case that already has cut-out for the m.2 module? And does the case have the PSu sitting over the CPU, limiting the size of the CPU cooler? Many guys go with water cooling in smaller cases but since good ones usually start at $100 why not go with an air cooler and better fans, or put the money towards more memory or a faster processor? Always get the fastest processor made and you can afford. (While you may not be able to afford a $1000 i7 you can probably afford a $350 i7, for example, or the fastest i5 made, which costs about $250.)
Me, I've had good luck with my ASRock mobo, so I wouldn't be adverse at looking at the ASRock Fatality Z170 Gaming-ITX/ac board since it has on-board HDMI 4k@60HZ output, along with 4K@60HZ DP 1.2, an Intel i219-v NIC, and can accept a single 2x16G kit of G.Skill F4-2400C15D-32GVR (~$130) or the G.Skill Trident F4-3000C15D-32GTZ (~$160). It can also accept 2x16G of 2133 and 3000, besides the G.Skill 2400s mentioned above. It's one of the few boards that can accept 32GB at 2133, 2400, 2666, 2800 and 3000 (the G.Skill 3200 kit is of 4 modules, though, and the Corsair 16GB module limits you to single modules which may, or may not, work since you will be installing 2 kits of 1 module each. And since some complain that the rated speed can't always be reached, why bother paying extra for it? I would rather get 2x16G @ 2133 than 2x8G @ 3200. ymmv.)
