The Apple control panel keeps care of what sense pins in the Monitor connector tell about the resolutions the monitor supports. So you can choose only resolutions the control panel suggests. Try using modifier keys. Some settings are available only in combination with the option key.
In case you want to check any possible resolution the graphic card is capable of, get the extension ActivateAllResolutions. It allows for choosing any possible resolution. This is useful especially for promiscuous use of a PowerBook's external graphics adaptor in various working environments (keeps you from reconfiguring the Mac-VGA adaptor all the time).
Be warned: ActivateAllResolutions also allows to render the external desktop pitch black and blow some Monitor electronics ;->