If you want to enjoy high fps with high details on a 1440p screen you will need a SLI setup. A single card will barely reach 50-60 FPS if you go easy on the anti aliasing in newer titles so if youre unwilling to spend the extra $$$ for a multi GPU setup you might as well stay with a 60 hertz screen, although i would rather recommend getting 1080p@144hz because hertz and fps > resolution imho.
And just fyi. I am a SLI user for many years now ( I remember buying 2 GTX 8800s for crysis back then... God i am getting old)
While the amount of FPS you can get combined with high Details are awesome when it works you will often run into games that simply dont get SLI support, it depends alot on the developers of the game in question. Sometimes SLI will only work months after release when the developers finally bother to make a profile, sometimes it will never work.
Not to mention that you might need a better PSU and then theres the cpu lane issue which has started to rear its ugly head in recent games (no its not a myth...)
Im not going to go into more detail.
TLDR: You might end up spending a great deal more money than you thought.
https://youtu.be/rRyFGoDVr28
It's a video about the difference of 8x/8x and 16x/16x pci lanes. But its going to provide you a good deal of information about the FPS you can expect with a SLI setup for different resolutions. Expect about 60% as much fps with a single card. (Regarding lanes it gets interesting around 3m20s when they go for witcher 3, but thats whole different (even more expensive) topic)
Feel free to ask me questions via PM.