Performance wise, there's barely any difference between a 7700k and a 6700k, especially at the same clock. The 7700k has greater likelihood to hit higher clock (with sample out there getting to 5Ghz) but that's with an ATX MB and its full size power delivery system.
In term of OCing, you'd need not just a bigger investment in CPU/cooler but MB as well... an increase of ~$200+ compare to non overclocking config in MB/CPU/AIO cost,
especially when the unlocked K CPU can require 50% more power than their locked non-K counterpart (numbers taken from Toms' review of the Kabylake CPU). You'd need a greater investment as well as silicon luck to get high OC in mATX.
In term of case, I underestimated the length of the card... the Air 240 can't really fit ~300mm card without removing the front fans and then cabling would still cause more problems so there are hardly any white mATX case out there that can accommodate such huge graphic card (one that could is the
BitFenix Phenom but it's not quite suitable for AIO, it'd require extra fans to support an i7+R9 390 config and it'd place a hard limit on the length/depth of the PSU to accommodate long graphic card). The best bet is probably the
Fractal Design Define Mini C w/Window.
So, if you want 7700k+Z270+AIO:
PCPartPicker part list /
Price breakdown by merchant
CPU: Intel Core i7-7700K 4.2GHz Quad-Core Processor ($349.89 @ B&H)
CPU Cooler: Corsair H100i v2 70.7 CFM Liquid CPU Cooler ($96.99 @ Jet)
Motherboard: Asus STRIX Z270G Micro ATX LGA1151 Motherboard ($203.98 @ Newegg)
Memory: Crucial Ballistix Sport LT 32GB (2 x 16GB) DDR4-2400 Memory ($184.99 @ B&H)
Storage: Crucial MX300 275GB 2.5" Solid State Drive ($83.35 @ NCIX US)
Storage: Seagate Barracuda 2TB 3.5" 7200RPM Internal Hard Drive ($59.99 @ Newegg)
Video Card: Sapphire Radeon R9 390 8GB Nitro Video Card
Case: Fractal Design Define Mini C with Window MicroATX Mid Tower Case ($79.99 @ NCIX US)
Power Supply: EVGA SuperNOVA G3 550W 80+ Gold Certified Fully-Modular ATX Power Supply ($91.99 @ SuperBiiz)
Operating System: Microsoft Windows 10 Home Full - USB 32/64-bit ($106.99 @ SuperBiiz)
Total: $1258.16
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2017-01-11 10:41 EST-0500
***SK hynix SL308 is temporary replaced in the list because it's out of stock at the moment.
If you have a spare USB laying around, you can look to get an W10 OEM license instead of this USB+retail license version and create your own USB installation drive, saving yourself ~$15 with the bit of extra work. It's just the convenience of not have to put any more effort into the final part of the build, since a mATX build would take more time to put together nicely.
You could also opt for the older but still very good
SuperNOVA G2 instead of the updated G3 to stave off another $20.
Non-K Kabylake, white innards:
PCPartPicker part list /
Price breakdown by merchant
CPU: Intel Core i7-7700 3.6GHz Quad-Core Processor ($309.69 @ SuperBiiz)
Motherboard: MSI H270M MORTAR ARCTIC Micro ATX LGA1151 Motherboard ($109.99 @ B&H)
Memory: Crucial Ballistix Sport LT 32GB (2 x 16GB) DDR4-2400 Memory ($184.99 @ B&H)
Storage: Crucial MX300 275GB 2.5" Solid State Drive ($83.35 @ NCIX US)
Storage: Seagate Barracuda 2TB 3.5" 7200RPM Internal Hard Drive ($59.99 @ Newegg)
Video Card: Sapphire Radeon R9 390 8GB Nitro Video Card
Case: Fractal Design Define Mini C with Window MicroATX Mid Tower Case ($79.99 @ NCIX US)
Power Supply: EVGA SuperNOVA G3 550W 80+ Gold Certified Fully-Modular ATX Power Supply ($91.99 @ SuperBiiz)
Operating System: Microsoft Windows 10 Home Full - USB 32/64-bit ($106.99 @ SuperBiiz)
Total: $1026.98
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2017-01-11 10:43 EST-0500
P.S: Can't believe in the few hours between post, the B150M Mortar Arctic become discontinued and unavailable on all sites.