Actually I had made a mistake (before the last tests I was playing with nvidia-settings and activated Xinerama and nvidia-settings modified the Xorg.conf and I didn't had direct rendering so the suspend works only without direct rendering
So, I have to choose:
1)do not use direct rendering: no accell, no compiz, but hibernate
2)use direct rendering: compiz, but not hibernate (or I need to restart X after the first hibernate cycle in order to be able to suspend the second time).
Nobody can help ?