Thanks Lisandro, Well I install the meta package xserver-xorg, and it install a lot of package and destroy my xserver configuration :)
After making a dpkg-reconfigure -phigh xserver-xorg in a console the XWindows works again and now I have the posiblity to enable the effect. I ask you for the name of the package because using the apt-cache I have 2 possiblity under my point of view: xserver-xorg-video-radeon and xserver-xorg-video-ati Well evrything works fine now =) a lot of thanks, GUAN
