What if I have an old 386 PC IBM computer and feed the output to a modern LCD monitor (say the game is outputting 420 x 320 or whatever was as standard display back then and playing that on a modern LCD running 1900x 1080). Would the game show up on the monitor, or would I have to buy an old monitor too to see and play the game?

I'm stongly considering buying an old 386 to run old timey flight sims. Back in the day the monitors were CRT and had to have the monitor setup for 400x 320, 640x480 or 600 x 800 etc. These old monitors used the same plug the standard DSUB so called VGA connector, same as one of the inputs that most new monitors also have.

EDIT: Basically what I want to know is running and old game on a 386 PC: Would it look as good on a new modern monitor as it did back with an old time vintage monitor, or would it look better running on the old monitor than it would on a new monitor?

I would think it would look terrible on a LCD. LCDs need to run their native resolution or they look like crap. A CRT would be the way to go and you can probably get one for next to nothing.

I have not tried my LCDs with my old 486 (which is mothballed, but workable). I do run some of my old DOS games using DOSBox on my modern machine, and they run at low resolutions from that era (320x240 for some). They look quite acceptable, IMO, on my LCD. The image quality doesn't prevent me from enjoying my game play.

As to which would look "better"... can't say. I no longer have a CRT, nor the space to build such a configuration if I did.

EDIT: One point which comes to mind, is whether a modern LCD supports those very low input signals like 400x300, etc. As I mentioned, I'm running DOSBox, and I'm not certain how it handles the video. Does it send the actual resolution I set to the monitor? Or is it faking something? No idea.

New LCD monitors will handle those low resolutions fine. Just be sure you have the monitor set to scale up otherwise the video will be quite small. It might look a bit less crisp than a CRT, but the res is so low it wouldn't be much. Your PC's POST screen is very low res don't forget.