The only reason to buy a i7 7700K right now is to play at 144HZ+, if you dont want to get an R7 1700, I would stay with your CPU and upgrade to some 6/8 cores later, your CPU is still great. If you dont want to spend the whole budget, but want to increase in gaming performance, I would sell your R9 390 while still miners are buying, and get some GTX 1070/1080.
PS: Since you are using 1080p 60HZ, there are no reason to change your PC for gaming right now. But for editing you can get a R7 1700.