Do I need G-sync or a new GPU?

[Kllown]

Looking into g-sync monitors, obviously would go with the Asus rog swift, but I would like an assessment of my rig to see if it's the best upgrade right now:

CPU: i5 4690k 3.5khz (OC to 4.2ghz)
GPU: Nvidia GTX970
RAM: 8GB
PSU: Corsair 750W

I would be buying the 1440p version of the screen as I'm already running at that, and my plan would be to sli my 970 after buying the monitor, but that could take a month or 2... Will it be worth buying the monitor first when I'm averaging 50fps on most games, or will the monitor just sync up to that speed (not really sure how the technical side of refresh rate and FPS work together)...

Yeah any advice would be awesome!?

 

[Kllown]

Or I could just get a 4k HDR monitor and upgrade my GPU (which sounds like a big step up for this setup but if I'm already spending big bucks on a monitor, why not?... Would that be an option)?