External display is difficult using a Netbook both on XP and Linux. Default Gnome manager has often difficulties to switch resolutions or manage clone display with different screen resolutions.
You can use the software URandRhttp://www.albertomilone.com/projects.html
It is a gtk one, manage saved profiles... And is not currently maintained (the author directly contributes to the standard gnome manager) and it is bugged... :-), but less bugged and more powerful than the default gnome one. It is the best one I had found!
You can stop internal or external display, manage well different resolutions, automatically update the virtual resolution, apply automatic or fixed resolutions. However take care that at the opposite of XP, in case the internal display is turn off, it is not restored automatically when you disconnect or turn off your external display. Even worse, the external display is disconnect too! (Thus, the two screens are black...). As long as you take care of this, it is OK. I is generally less problematic to turn on you external screen after the notebook is on.
There is also a good page about xorg configuration and different gtk GUI.http://www.thinkwiki.org/wiki/Xorg_RandR_1.2#GUIs