In order for us to fully gauge if you're making a sound move, you should list your specs to the system you currently have. List them like so:
CPU:
Motherboard:
Ram:
SSD/HDD:
GPU:
PSU:
Chassis:
OS:
Including the age of the PSU(apart from it's make and model).
Where are you located, how much will you want to spend on your upgrade path and where would you like to purchase the parts from(preferred site for purchase)?
CPU: i5-6400 @ 2.70GHz
Motherboard: MSI B150M Mortar
Ram: 4 x 4GB HyperX Fury DDR4 2133MHz
SSD/HDD: 1 x Samsung 840 EVO 250GB SSD, 1 x Samsung 850 EVO 500GB SSD, 1 x Western Digital WD Green 1TB 7200 RPM Hard Drive
GPU: Nvidia GeForce GTX 1070
PSU: Antec 520W High Current Gamer PSU (Got the power supply at the same time as the GPU so I believe I got it in 2016)
Chassis: Not sure about the case its very old from around 2010 and I've never looked at getting a new one
OS: Windows 10 Pro 64-bit
I'm located in Perth, Western Australia so I'm looking to buy from MSY, Austin Computers or PC Case Gear. I looked up that the i5-9600KF was the best budget CPU for gaming so I'm only looking to spend around $300 for that CPU and probably around another $150 or so for a new motherboard.