Like Diablo 2 for example. Is the resolution true or is it distorted at all? I'm asking because people say that playing 720p console games on a 1080p monitor looks bad. So what about classic PC titles from the 90s and early 2000s like Diablo and Baldurs Gate, etc? Are they displayed properly with High Res monitors?
No, if the game is 3d( not 3d glasses) then they can scale to any resalution, but an old game like diablo is 2d graphics so they won't be any extra detial in the game, hense it will be lock to a resalution.
It's actually down to the game - Freelancer for example won't do higher than 720p. The original Godfather won't do above 1680x1050, although that actually doesn't look too bad on a 1080p monitor (whereas 720p does look pretty bad).
And as for DOS games, you're not going to get 1080p :-) You can play 320x200 Heretic or Doom upscaled though and after a bit you get used to it and just enjoy the game! For very old Windows stuff, I'd hugely recommend Planescape Torment or especially Baldur's Gate 2 if you haven't already played them.
Polygons render natively at any resolution but pixel-art does not. So if your game was created in a 3d game engine it will render properly but with aliasing (which you can combat with anti-aliasing). But if the game used pixel-art, there isn't much you can do aside from some custom blitters that people made for ePSXe years ago that I wish someone would adapt into a program to work with older PC games.
it depends on what options are available in the screen rez selection menu either in game or on the games config menu on the installer or in the main install folder...
some older games only run at 4/3 or 5/4 res such as 1024/768 or 1280/1024
games like need for speed most wanted for instance later got patched to run on 16/9 monitors but that was a community patch not all older games got them.
most games from the 2003 onwards to 2007 supported up to 1680/1050 then after that most new games supported 1920/1080-1200. pre 2003 your likely to be stuck at 1280/1024 max. unless like i say the game got an updated rez patch...
some games like f.e.a.r just left it up to the gfx card and would run any available rez but if you went over the standard for the day 1280/1024 you would see a message saying the game was not optomized to play at this rez and may cause issues. it rarley did as f.e.a.r was well written (well after 7 patches ).