simply turns on only the external VGA:
xrandr --auto --output VGA-0 --mode 1680x1050 --output LVDS --off
turns on both monitor, the external widescreen monitor is right of the notebook:
xrandr --auto --output VGA-0 --mode 1680x1050 --right-of LVDS
turns off the external monitor, only the internal is still on:
xrandr --auto --output VGA-0 --off
returns to "classic" clone mode. The "--auto" on all those commands turns on all
available monitor, so that the system is in an defined state:
xrandr --auto --output VGA-0 --mode 1024x768 --same-as LVDS
source: http://en.opensuse.org/Multiple_Screens_Using_XRandR
