GVidChanger-minimal (gvidm) is a GTK app to quickly and easily change video resolutions in X. Running it will pop up a list of available modes; upon choosing a mode or cancelling, it exits. If you are running dual or multi-head displays, it will give you a list of screens so you can select the appropriate one.
You can also specify on the command line a mode to switch to immediately, or a list of modes to choose from.