I think your new friend is xrandr
Just try to type it in the terminal, with your external monitor connected.
If it works as it is supposed you'll have a list with supported res. aso.
To connect your extra monitor some command like this would probably do it
xrandr --output VGA --auto --left-of LVDS
If you try to connect via VGA, and want to have a dual desktop, that is.
Another example is
xrandr --output VGA --mode 1280x1024
But the best way to make a good setup is to try and experiment.
An alternative is to change your xorg.conf, just to add the extra monitor there.
(Needs a bit more configuring)