Well as 1070 owner i can say its not enough for me at 1440p 144hz no gsync, but it all depends on what games are you going to play and what do you expect.
Gsync can solve actually tons of your problem, but to be honest i dont know how it looks like at 45-50 fps, and thats (and lower) what you will see pretty often if you wanna play last years releases max/high graphics. So to get a correct answer you should write more about your plans, but for me - 2080/1080ti with gsync (or gsync compatible if itll work good) will be very nice.