Thanks to those who have posted helpful suggestions. I tried installing the latest Nvidia driver using the x-swat ppa. I also tried installing the latest driver directly from Nvidia (295.20). The latest driver was not entirely satisfactory. What DID work was installing the nvidia-current driver from the standard repos (version 280.13); this driver also has the necessary support for my molecular modeling program (YASARA-Structure). The 295.20 driver worked, but it produced problems noted elsewhere, such as blacked-out areas within application windows. The 280.13 driver seems to be behaving flawlessly. Note that I am using a somewhat dated system (Dell XPS 410 with Intel Core2Duo, Nvidia GeForce 7900 GTX).
The instructions that helped enormously can be found at the following site:https://help.ubuntu.com/community/Binar ... wto/Nvidia
You will get a black screen when rebooting after installing the driver. The key to getting the driver to work is as follows:
--Switch to the console by using ctrl+alt+F1, or reboot and select recovery mode from the GRUB menu.
--Open and edit xorg.conf like this: sudo nano /etc/X11/xorg.conf.
--Find the line that says: Section "Screen"
--Insert a new line that says Option "UseDisplayDevice" "DFP".
--Save the file. If you had to restart into recovery mode, type reboot, otherwise restart your display using sudo /etc/init.d/gdm restart.
My situation was complicated by the fact that I am using 2 Dell 2407 monitors that are shared with my Mac. The Mac uses the left monitor as primary in DVI mode and the right monitor in VGA mode. The linux PC uses the right monitor as primary in DVI mode and the left monitor in VGA mode. After getting out of the black screen, I was able to access the Nvida Xserver settings to get my dual-monitor configuration set up and saved to the Xorg.conf file.
I still have an issue of the Nvidia settings not being faithfully remembered when rebooting. The system tends to disable the left monitor that is in VGA mode. However, this is easily remedied by using the Nvidia Xserver settings.
It seems that some others have had problems getting Nvidia drivers to work. I hope that future versions of linux and Nvidia drivers will make it easier to install the drivers effectively.
As for now, I would mark this problem as SOLVED.