You should spend as much money as possible, on an 8086K 5.3GHz binned with a top of the range Z370 motherboard, and 16GB/32GB DDR4-4266MHz RAM to get the best performance. If you are playing at 720P/1080P then you probably should make sure you have a 1080 Ti as well otherwise you might find you dip below 160 FPS in some games at Ultra settings, I wouldn't recommend SLI however since the benefit isn't there really. If you get yourself a nice 1TB Samsung 970 Pro M.2 drive, your games could load way, way faster too, up to 20%+ in some titles.
It's going to be a massive upgrade if you are coming from an i7-2600K etc. you'll probably get a 15-20%+ performance boost, and in my opinion spending about £1300-1500 for this gain is totally worth it.
I doubt you'll get any better advice than that, since nothing can touch what I have recommended. Make sure you post some photos once you've built it.