Your graphics card software may already have a utility to do that. For example, the NVidia drivers come with a control panel, and you can easily change the settings from there (system tray).
You can probably find others with google, but here's a freeware utility to do what you want:
http://www.codeproject.com/tips/resswitch.asp
It's a command line utility that can be run as a desktop shortcut.
SD