See xorg.conf below:
# /etc/X11/xorg.conf (xorg X Window System server configuration file)
#
# This file was generated by dexconf, the Debian X Configuration tool, using
# values from the debconf database.
#
# Edit this file with caution, and see the xorg.conf(5) manual page.
# (Type "man xorg.conf" at the shell prompt.)
#
# This file is automatically updated on xserver-xorg package upgrades *only*
# if it has not been modified since the last upgrade of the xserver-xorg
# package.
#
# If you have edited this file but would like it to be automatically updated
# again, run the following command:
# sudo dpkg-reconfigure -phigh xserver-xorg

Section "Files"
Fontpath "/usr/share/fonts/X11/misc"
Fontpath "/usr/share/fonts/X11/cyrillic"
Fontpath "/usr/share/fonts/X11/100dpi/:unscaled"
Fontpath "/usr/share/fonts/X11/75dpi/:unscaled"
Fontpath "/usr/share/fonts/X11/Type1"
Fontpath "/usr/share/fonts/X11/100dpi"
Fontpath "/usr/share/fonts/X11/75dpi"
# path to defoma fonts
Fontpath "/var/lib/defoma/x-ttcidfont-conf.d/dirs/TrueType"
EndSection

Section "Module"
Load "i2c"
Load "bitmap"
Load "ddc"
Load "extmod"
Load "freetype"
Load "glx"
Load "int10"
Load "vbe"
EndSection

Section "InputDevice"
Identifier "Generic Keyboard"
Driver "kbd"
Option "CoreKeyboard"
Option "XkbRules" "xorg"
Option "XkbModel" "pc105"
Option "XkbLayout" "us"
EndSection

Section "InputDevice"
Identifier "Configured Mouse"
Driver "mouse"
Option "CorePointer"
Option "Device" "/dev/input/mice"
Option "Protocol" "ImPS/2"
Option "ZAxisMapping" "4 5"
Option "Emulate3Buttons" "true"
EndSection

Section "InputDevice"
Driver "wacom"
Identifier "stylus"
Option "Device" "/dev/input/wacom"
Option "Type" "stylus"
Option "ForceDevice" "ISDV4"# Tablet PC ONLY
EndSection

Section "InputDevice"
Driver "wacom"
Identifier "eraser"
Option "Device" "/dev/input/wacom"
Option "Type" "eraser"
Option "ForceDevice" "ISDV4"# Tablet PC ONLY
EndSection

Section "InputDevice"
Driver "wacom"
Identifier "cursor"
Option "Device" "/dev/input/wacom"
Option "Type" "cursor"
Option "ForceDevice" "ISDV4"# Tablet PC ONLY
EndSection

Section "Device"
Identifier "nVidia Corporation NV31 [GeForce FX 5600]"
Driver "nvidia"
Busid "PCI:2:0:0"
Option "AddARGBVisuals" "True"
Option "AddARGBGLXVisuals" "True"
Option "NoLogo" "True"
EndSection

Section "Monitor"
Identifier "DELL E173FP"
Option "DPMS"
EndSection

Section "Screen"
Identifier "Default Screen"
Device "nVidia Corporation NV31 [GeForce FX 5600]"
Monitor "DELL E173FP"
Defaultdepth 24
SubSection "Display"
Depth 1
Modes "1280x1024" "1152x864" "1024x768" "800x600" "720x400" "640x480"
EndSubSection
SubSection "Display"
Depth 4
Modes "1280x1024" "1152x864" "1024x768" "800x600" "720x400" "640x480"
EndSubSection
SubSection "Display"
Depth 8
Modes "1280x1024" "1152x864" "1024x768" "800x600" "720x400" "640x480"
EndSubSection
SubSection "Display"
Depth 15
Modes "1280x1024" "1152x864" "1024x768" "800x600" "720x400" "640x480"
EndSubSection
SubSection "Display"
Depth 16
Modes "1280x1024" "1152x864" "1024x768" "800x600" "720x400" "640x480"
EndSubSection
SubSection "Display"
Depth 24
Modes "1280x1024" "1152x864" "1024x768" "800x600" "720x400" "640x480"
EndSubSection
EndSection

Section "ServerLayout"
Identifier "Default Layout"
screen "Default Screen"
Inputdevice "Generic Keyboard"
Inputdevice "Configured Mouse"
Inputdevice "stylus" "SendCoreEvents"
Inputdevice "cursor" "SendCoreEvents"
Inputdevice "eraser" "SendCoreEvents"
EndSection

Section "DRI"
Mode 0666
EndSection

Add HorizSync and VertRefresh lines to the monitor section. They should reflect the capabilities of your monitor.

Example (for my monitor)