At first I thought you bought a 4K monitor with existing 1080's. But they are all 1080. I don't think you will notice without gsync. You can turn if off with your mentioned new monitor. To use it, you would need to play on that one monitor.
I just bought a Dell 27in 4K monitor on a price match for $10 less than your amazon priced XL2420G at $529.
http://www.amazon.com/BenQ-G-SYNC-24-In ... B00MUT6SLEhttp://accessories.us.dell.com/sna/prod ... u=210-ADOFThat is a lot of money for a 1080 monitor, but you get to do whatever you wish
Why did you get 2 980's? not for 1080 gaming is it?