SetsulYeah, PSU of course. Like I said a mobo for the 1920X would be full ATX any so you'd have plenty of slots.
SLI is the software side of multi GPU setups. If you have 2 GPUs, but SLI no enabled then one will be turned off.
If SLI is turned on, but the game does not support then you get nothing again. Still only using one GPU, 2nd one is turned off.
If SLI runs like shit then you get less fps than without it.
In most new games it'll run poorly and you'll get something between 0 and 30% more fps than with a single GPU.
If it runs well then you get 50-70%. That's it.
It used to get better as time went on and drivers improved, but since that meant SLI might actually end up being useful nVidia decided to disable it on everything below a 1070. Wouldn't want anyone to buy two cheaper GPUs that combined end up getting you the same fps as a GPU that's more than twice as expensive. So I wouldn't put much faith in nVidia spending a lot of time and effort on improving SLI scaling these days.
Yes, looks smoother, still feels exactly as choppy (or worse) as with a single GPU.
So what that means is that, for example, you could turn off SLI if you knew that the game you're playing does not/performs worse with SLI? And by the sound of it it's like a software option, rather than having to manually remove a graphics card to disable SLI?
Is SLI worth buying then, if newer games perform worse with it? In that respect, how is SLI much different to just using two graphics cards? As in, if you want to run a game without SLI support at 60fps that can't reach it on just a 1080 Ti, how do you improve graphics without using SLI?
E: Back to the build, would it be possible for you to find me the motherboard and PSU to use? I want to try and work out how much of an impact the price of the mobo/CPU will be on the build so I can see if the 4K monitor is affordable.