Excellent information. For Mac users: go to your Monitors and Sound control panel (click on the left icon labeled "Monitor" if it isn't already highlighted) or, if you have the control strip showing, click on the module that looks like a checkerboard in a frame. Either will show your available resolutions. Pick the one you want.

You will get a message that says something like "You have changed your screen resolution. Click OK to confirm." If you don't click within a few seconds, your screen will return to the previous resolution. That avoids the problem kanebear referred to at the end of his post - if your monitor won't work at the new resolution you won't see the message, so you can't click OK and your screen won't change.

That's how it works through System 9 and sub-releases. I haven't used System X yet.