How to setup my screen Samsung 2032BW in ubuntu 12.04

[smoitel]

I tried to setup my screen with no success for the moment. I just have the 640*480 resolution with ubuntu 12.04. I changed the xorg.conf. I tried with xrandr and I always get this message:
"Failed to get size of gamma for output default".
I really don't find what I can try more.
Thanks for help !