Encoding video takes a lot of work, regardless of where that work is done. If your video card has a dedicated section of silicon that does nothing else anyway, then that's the place to do it, so that the CPU doesn't have to. If the CPU does have to, then that's a massive additional load that it might not keep up with in addition to everything else.
x264 is the CPU. Every rig has that option, because it's done in software, with that massive additional load. If you have a monster CPU, that might be okay. Otherwise, avoid it.
NVENC is the GPU. Specifically, a dedicated section of that GPU that does nothing else anyway. If you have it, USE IT! Seemingly no additional system load because it's using a dedicated physical machine that does nothing else anyway. Just keep it from overheating, don't overload its settings, and you're good!
As for the settings themselves:
OBS -> Tools menu -> Auto-Configuration Wizard
