mebcitadel :
Ok that all makes sense so with that logic then, If I consider 45fps @ ultra 1080 p "butter smooth" with no g-sync, would I be just as happy with a 60hz monitor and g-sync? I ask because if that's the case I'm buying a 4k monitor and another 980 Ti like right now.
Honestly, it all depends on the person. Some people have a hard time...or are unable to see a difference between 1080p (or even 720p) and 4k. Just like there are some people that can't tell a difference between 30 fps and 60 fps...much less 60 fps and 120 fps.
Depending on the game and gamer having a 4k 60 hz display is a lot better than 120 hz 1080p display. And if you can add in g-sync and other special features, then the monitor will get even better.
Depending on the game settings and resolution you use with any particular setup will dictate the average, low, and max fps you can get. If you are used to getting 40 - 60 fps in games at 1080p...then adding in another GPU to run in SLI will definitely help increase your fps. And by switching over to 4k gaming, you should end up similar in fps (once you switch down AA since it is not really needed at that resolution).
So, my honest opinion is yes. Go SLI and get a 4k setup. I have seen some in stores but not upgraded myself yet since I am buying a house and still have to buy a lot of stuff for it. You can look up opinions that people have for and against each side of the argument, but it all comes down to your personal preferences.
If possible I would recommend you go to a local store and see a 4k display for reference though, as well as turning down your game settings to see the 144 hz refresh rate in action. That way you can get a basic comparison of what you can see yourself.