I have xfce4 installed on ubuntu 12.04. When I have a laptop connected to an external monitor.
When i go to Display and choose the resolution of the external monitor nothing happens..
I don't have any problems with default Ubuntu Display.
Hey, normaly it's very easy to do those settings with the tool "arandr". But i don't know if thats the way to go in xubuntu, but under Debian this works.