Select a game exclusively for a single monitor gaming

There are several issues. 27 or 24. Graphics as a priority, it's probably IPS. Do I need to take with 2K and 4K, or software configured to do a pseudo higher resolutions (DSR) in the panel NVIDIA? Since online games with the kids can not stand to play, whether a lot of Hertz, the more that something is not too clear. For example, why 240 Hz, if the game can squeeze a maximum of 120 fps. Correctly I think, that in this case is 120 Hz? When well then, I will give 240 Hz? Only when more than 240 fps? I apologize in advance - I'm not a techie. And finally, the notorious G-Sync! Solid laudatory odes, but very confused how much this joy is worth. Now on a normal Monica 24Asus for 8000 R. can play 2K and 4K , but I don't know whether this is a pseudo high resolution from the present hardware.(just not seen). Stupid analysis, that according to my understanding all right, then I would stay at AGON AG271UG AOC 27 4K. But it's not for NVIDIA. Then AGON AG271 QG AOC 27 165Гц 2K G-Sync IPS 4 ms 350 CD/sq. m FlickerFree. Please give advice on the issues raised and possible specific recommendations. Of course, I understand that ASUS ROG is great, but 65 - thousand too much!.