1. For a Gaming Box, the Intel CPus are the better choice. The 1700 is almost $200 more than the 7600k which performs better.in gaming. If thinking AMD wait for April 11 and see what the new CPUs can do. Entry point for MoBos is higher too
2. You'd want a Z270 MoBo,... unless you have a spare Skylake CPU laying around for which you could use to upgrade the BIOS so it could take Kaby Lake.
3. Which Giga Z270X ? Be aware that the low - mid range boards will saddle you with low budget audio solutions. Asus was thge only one doing this w/ Z170 but w/ Z270, the other brands are following suit. We agreed with anandtech that the MSI Z170A SLI Plus set a new baseline standard at the $130 price point.
http://www.anandtech.com/show/10236/the-msi-z170a-sli-plus-review
This continues with the Z270 version which delivers ALC 1220. Most options in this price range offer last generation, low budget sound solutions (ALC 887 / 892).
From Gigbayte, cheapest option AFAIK is the Gigabyte GA-Z270-Gaming K3 ($140)
So unless workstation type apps are a significant part of your activity, i just don't see the 1700 as an appropriate choice, and especially not at a cost premium of $200.