xrandr and session

[Jean-Christophe]

Hello to all.
Here is my problem:
I've got a laptop, and when I plug a screen on it, I use xrandr to 
switch my resolutions. This works fine.
But xfce doesn't reconize it, and I have then my 800x480 desktop 
(eeepc...) shown on my 17" configured on 1600x1200 and all the other 
parts are blank. I can move my cursor on it but xfce refuses to display 
anything there.
One solution I found is to do: 'killall xfwm4 & xfwm4'. Then xfce 
reconizes my desktops but xfwm isn't launched with my user's preferences 
and can't be configured.
How can I make xfce reconize my xrandr changes?