I just did a bit of testing with zDoom. It will not run ANY fullscreen resolution other than my native. I tried numerous 4:#, 16:9, and 16:10 settings and the only one that worked was 1680x1050. If I highlight a resolution and press T to test it for 5 seconds, the game minimizes to windows for 5 seconds.
All resolutions and aspect ratios work in windowed mode.
ok, this is weird. I just ran WoW and set it to fullscreen 800x600 and it did the same thing. I've installed games and demos since getting my LCD monitor and had to change the resolution to 1680x1050. So either it's just some games, or it has something to do with my system or drivers. I can't think of anything I've changed recently except video drivers, and that was in an effort to fix this issue.
EDIT: I just loaded up MOHAA, which I haven't played in years, and I've been copying my install from computer to computer instead of reinstalling. So it's set to 1024x768. It loaded up fine, and I just played for a few minutes at 1024x768. Though it's a quake 3 engine game, which I think was OpenGL (isn't OpenGL dead now and pretty much everything is Direct3D?) so maybe it works differently.
I tried Giants: Citizen Kabuto, which I just bought the other day, and haven't run until now. It's running 800x600 fullscreen just fine.