If you have the 980Ti you should definitely be looking at upping the resolution. You haven't got the power for 4k, but 1440p should be a reality. It's what I have.
For g-sync the problem is you don't actually realise when it's working, or not. No point if your monitor can only do 60Hz, as chances are the card is already faster than that, at least on most current games, even in ultra settings. But if you get a 100Hz or even 144Hz refresh rate monitor then g-sync could be good.
Of course, refresh rate may not mean much if the response time of the monitor is pitiful. Generally you need a TN panel for the fastest response in milliseconds, but the colour purists would argue that TN panels aren't as good as some less responsive types.
I have this one : https://www.overclockers.co.uk/dell-s2716dg-27-2560x1440-tn-g-sync-144hz-gaming-widescreen-led-monitor-midnight-grey-mo-084-de.html
but that is now nearly £250 more expensive than I paid back in 2015