I found workaround for my problem. I suppose it's not related to xorg or nvidia drivers.
The main cause seems to be that I still had legacy grub from Ubuntu version 9 not updated to grub2. Because of that, the running kernel version (2.6.31-20) was not the latest istalled (2.6.32-22); grub-update had no effect on grub menu and nvidia drivers package did not care about old kernel.
I found workaround for my problem. I suppose it's not related to xorg or nvidia drivers.
The main cause seems to be that I still had legacy grub from Ubuntu version 9 not updated to grub2. Because of that, the running kernel version (2.6.31-20) was not the latest istalled (2.6.32-22); grub-update had no effect on grub menu and nvidia drivers package did not care about old kernel.