Why not use the Nvidia proprietary driver?
Instead of choosing a lower resolution, you should lower your "render scale". That way there will be no mode switching when you go to the desktop and the GUI will remain at native resolution.
Also, even if you set Max FPS to 60, we recommend that you select Uncapped FPS mode because it works better than the old Doom 3 capped mode.
See also:
https://wiki.thedarkmod.com/index.php?title=Performance_Tweaks