VietGuy is right.
Depends on your budget however. 1080 is good for 1440p, but if you have a pretty decent sized budget, a 2080 or 2080ti would not hurt. But if a limiting budget exists, just stick with 1080.
From my tests and what I have seen, SLI is not a very worthwhile upgrade. Support is decreasing, but if you get a very good deal on a 2nd hand 1080 and have a spare SLI bridge, then you can consider it.