I5-8400 is a very good gaming processor.
One rule of thumb for a balanced gamer is to budget 2x the cost of the cpu for the graphics card.
I think you are spot on there considering GTX1060/GTX1070.
If you favor fast action shooters, look to a stronger graphics card.
If you favor mmo, sims, and strategy games, a lesser card will do.
Some comments on your parts list:
1. I5-8400 can not be overclocked and comes with a perfectly adequate stock cooler.
If you want a quieter cooler, the $35 scythe kotetsu is a much better cooler.
Here is a review:
http://www.silentpcreview.com/article1391-page1.html
2. 120gb is too small for a windows C drive. Many things default to the C drive, and as it gets filled, it will lose performance and endurance.
Buy 240gb at least. You can defer on the hard drive until you actually need space. If you can manage 500gb, you may never need a hard drive at all.
I would pay a bit extra for Samsung EVO which seem to perform better and be a bit more reliable.
3. If you value fancy heat spreaders and "bling" then the vengeance ram is OK. I would buy the cheapest low profile ram if the price is sufficiently lower.
4. A 550w psu is sufficient to run even a GTX1080.
If the price is not too different, I would buy a 550w Seasonic focus for the better quality.
https://www.newegg.com/Product/Product.aspx?Item=N82E16817151203
$55 after rebate.
5. What will be your case?
3. If you do buy a hard drive, look at WD, they seem to be a bit more reliable.