Here we go. photo of my dumpstered PC monitorhttp://i.imgur.com/Us38JKQ.jpg
And now for some TL;DR-worthy rambling:
The settings I'm using for Mars Matrix is (game settings) double vertical resolution to 448, then horizontal resolution to 612 (because some post on shmups forum recommended this), then stretched (monitor settings) to an aspect ratio that feels about right. A little research uncovered that the native aspect ratio of CPS2 games is 12:7, but then the games are designed to be squashed to 4:3 resolution, so that would theoretically have the most logical response from the perspective of control, However it felt a little weird because the default settings gives you a slightly wider screen, so I'm not used to 4:3. Also, I still don't know what aspect ratio the monitor is. It might be 16:10, but maybe it's something more exotic like 3:2, but in any case, it feels good and looks more or less alright.
Also tried crazy taxi as a test of something that's actually 480p and not stretched 224p. That looked pretty nice. I like the clarity that the VGA adapter gives stuff, and it's nice that you can warp the image without it looking shitty. Also: No input lag!
I think I ought to try Guilty Gear X out, because that game should look awesome through VGA on a monitor that has good support for 480p.