Note that installing the nVidia driver will automatically fix some screen resolution problems.
# Install the driver (only if you have an nVidia grapics card):

sudo apt-get install nvidia-glx

# Enable the driver:

sudo cp /etc/X11/xorg.conf /etc/X11/xorg.conf.backup.nvidia
sudo nvidia-glx-config enable

If there was an error message, you might have to enable the driver manually as follows:

sudo gedit /etc/X11/xorg.conf

Replace:

Section "Device"
    ...
    Driver        "nv"
    ...
EndSection

with:

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

# (optional) Create a desktop entry for nVidia Settings (accessed via the "Applications / System / nVidia Settings" menu):

sudo gedit /usr/share/applications/nvidia-settings.desktop

Add the following to the blank file:

[Desktop Entry]
Name=NVIDIA Settings
Comment=nVidia Settings
Exec=nvidia-settings
Icon=
Terminal=false
Type=Application
Categories=Application;System;

# (optional) Disable the nVidia splash screen:

sudo gedit /etc/X11/xorg.conf

Change:

Section "Device"
    Identifier    "NVIDIA Corporation ..."
    ...
EndSection

to:

Section "Device"
    Identifier    "NVIDIA Corporation ..."
    ...
    Option        "NoLogo"    "true"
EndSection