NVidia supplies the nvidia-settings program with it's driver. It is a graphical program where you can configure a dual head system.
ATI has a similar program, but I don't remember it. You are able to export your current settings to xorg.conf. I don't know if it has been updated to the /etc/X11/xorg.conf.d/ scripts that newer versions of the Xorg server uses by default.
If you use KDE, there is a krandrtray program, which allows you to switch resolutions on the fly, via a program that sits in the tray. It is a graphical front end to the xrandr program.
Your desktop may also have a program described as "size & orientation" or "display" as well.
Last edited by jschiwal; 12-03-2010 at 11:47 PM.