Install nVidia graphics driver

Note that installing the nVidia driver will fix some screen resolution problems.

1. If you plan to connect multiple monitors, start out by connecting only the main monitor in order to keep things simple. (Once you have the driver working fine for just that monitor, you can then connect the other monitors and use the nvidia-settings utility to configure them.)
2. Now reset the X server configuration to very basic settings:

sudo dpkg-reconfigure xserver-xorg

You will be presented with a series of questions. Choose the default answers for all the questions except for the following:
1. Choose "vesa" for the X server driver.
2. Choose "No" for monitor autodection.
3. Choose "medium" for the method for selecting the monitor characteristics.
4. Choose "1024x768 @ 60Hz" for the monitor's best video mode. (Don't worry, we'll get better screen resolutions once we install and enable the nVidia driver.)
5. Choose "Yes" for writing the monitor sync ranges to the config file.
You can now optionally restart the Gnome Desktop Manager (GDM) for these settings to take effect before continuing. Alternatively, you can just continue the driver installation without restarting the GDM.
3. Install and enable the nVidia driver:

sudo apt-get install nvidia-glx-new
sudo nvidia-glx-config enable

Note: If you have a GeForce4 video card, you may need to use nvidia-glx instead of nvidia-glx-new. And if you have a very old card, like a TNT, you may need to use nvidia-glx-legacy instead.
4. Reboot the computer for these settings to take effect. (In theory, you could just restart the Gnome Desktop Manager, but I ran into a situation where, after an error, the nVidia driver didn't seem to load upon restart the GDM.)
5. Set up screen resolution, dual monitors, Xinerama, etc.:

sudo nvidia-settings

You can change the settings in the "X Server Display Configuration" section and then click the "Save to X Configuration" button and save the file as /etc/X11/xorg.conf.
6. (optional) Disable the nVidia splash screen:

sudo gedit /etc/X11/xorg.conf

Change:

Section "Device"
...
Driver "nvidia"
...
EndSection

to:

Section "Device"
...
Driver "nvidia"
...
Option "NoLogo" "true"
EndSection

7. (optional) Restart Gnome Desktop Manager for these settings to take effect.