I'm fairly sure this question belongs in this section, and although the search brings up many related topics vis-a-vis the use of resolutions, they're usually quite old/pre-version 3/pre-directX 9, and I've been reading in more and more topics about peoples' monitors not being able to run games in certain, smaller, classic resolutions (320x200), and the solutions for getting around this on the player-end seem horrifically complex, technical and off-putting to the casual player.
For maximum playability by default on modern machine and under modern circumstances, should I therefore actually use 640x400, with the contents merely scaled up double if I want to create the old-style look, or am I being needlessly paranoid about odd grey-and-black hatching on text boxes that I've seen?
Much obliged!
Since the filters were introduced, it's no problem at all to run a 320 game windowed or fullscreen.
Depending on the resolution, x3 is most likely the best option for windowed playing; while gfx cards shouldn't cause any trouble with x2 & fullscreen.
Yes, it's fine to make your game at 320x200.
However, before releasing it, it's a good idea to run Setup and turn on the 2x filter so that the game will actually run at 640x400 by default, which should avoid compatibility problems.