Main problem is the use or nvagp with nvidia driver. via-agp is conflicting, and it sems to be impossible to blacklist it. So I've removed directly the module on disk!

/lib/modules/2.6.22.6-desktop-3mdv/kernel/drivers/char/agp/via-agp.ko.gz

xorg.conf has

   Option "NvAGP" "1"

and modprobe.conf:

   options nvidia NVreg_EnableAGPFW=1

Now, suspend to disk works out of the box, with kpowersave.