So I've been playing an FPS game which gets around 70~80fps usually. Upgrading my graphics cards is not an option now, so I'm willing to buy a monitor with 144hz, but I'm not sure if it would make any difference because of the fps I'm getting now. From your experience, would it be worth in terms of gaming performance?
Well, it definitely won't increase your FPS :)
Seriously, for gaming performance it technically doesn't matter. The only difference is the amount of frames that are shown by the monitor, not the amount of frames rendered and frames you can do inputs on. If you do choose to get a new monitor, get one with FreeSync and GSync, since if your GPU will output less frames than the monitor supports, it can tear, and that looks really ugly (as if your OC is a bit too high), although this can be regulated through MAXIMUM RENDERED FRAMES in most games. Also look for the monitor response time if you want/ if your current monitor‘s is high(=slow).
I recommend not getting a new monitor in your case though,(unless you really want for other reasons), instead save that money for a new GPU.
As long as you get the same resolution, nothing will change in performance.
Though, if you match your graphics card with the monitor: AMD GPU + monitor with Freesync Premium / NVIDIA GPU + monitor with G-Sync, it can actually make your gameplay seem smoother/more pleasurable to your eyes, in games where your FPS may drop. Thanks to "low frame rate compensation (LFC)", your monitor will adjust its refresh rate to match your frame rate, so no tearing occurs.