vapour :
Upgrading ram is not as easy as it used to be. Buy another 8 gb stick may not work with the first stick. Just an FYI. GTX 1070 should handle that 1080p 144Hz monitor pretty well.
But not as well as a 1070ti. Also a ~5-10% disadvantage in games from the CPU as well. As new features get rolled out the trend is pretty solid towards always needing more gaming performance a few days/weeks/months in the future. No reason to start out slower to avoid upgrading ram. And upgrading ram has not gotten harder. It is the same as it ever was. You have to match times now and you did 20 years ago too. We do plenty of them on an ongoing basis.
The fastest gaming is the fastest video card with a fast enough CPU. The i5-8400 with the same GPU as a 1600 will beat it; but the difference will be greater when you compare a 1070ti vs a 1070.
Here is a recent (this month) review of multiple CPUs for PC gaming by PCgamer.
http://www.pcgamer.com/the-best-pc-gaming-cpus-processors/
Here is Tom's hardware's just published 1070ti review:
http://www.tomshardware.com/reviews/nvidia-geforce-gtx-1070-ti-8gb,5311-20.html