I like the I5-7600K for gaming.
The higher available clock rate is what most games need. More so than many cores.
To get that clock rate, you will need a Z270 based motherboard so you can overclock.
Most any will do.
As of 2/23/17
What percent of samples can get an overclock
at a vcore around 1.4v.
I5-7600K
5.3 samples exist, unknown % of occurence
5.2 13%
5.1 27%
5.0 52%
4.9 72%
You will need a decent cooler since the K does not come with one.
I use a noctua NH-U12s and have no problem with cooling on my 7600K @4.9.
A equally good cooler would be the $35 scythe kotetsu.
You will need 160mm available to install either.
GTX1070 is an appropriate graphics card.
I like EVGA as a brand and would pick the GTX1070SC model
It has a modest factory overclock for not much of a premium.
Buy a 2 x 8gb DDR4 ram kit. Speed is not important, 2400 speed is ok.
I will never again build without a ssd for the "C" drive. It makes everything you do much quicker.
120gb is minimum, it will hold the os and a handful of games.
But, many things default to the "C" drive.
When a SSD nears full, it will lose performance and endurance.
240gb is the recommended minimum.
If you can go 240gb, or 500gb you may never need a hard drive.
You can defer on the hard drive unless you need to store large files such as video's.
It is easy to add a hard drive later.
Samsung EVO is a good choice for performance and reliability.
Cases are a personal thing, bust your budget if you must to get one you love.
It will be with you for a long time.
My requirements would be two front 120mm intakes or better with a filter to keep parts clean.
Silverstone TJ-08E works well if you want a M-ATX motherboard.
GTX1070 needs only a 500w psu.
I have no problem overprovisioning a PSU a bit. Say 20%.
It will run cooler, quieter, and more efficiently in the middle third of it's range.
A PSU will only use the wattage demanded of it, regardless of it's max capability.
600w will run even a GTX1080ti.
Look to Seasonic for top tier 1/2 quality.
Here is a quality list:
http://www.tomshardware.com/forum/id-2547993/psu-tier-list.html