Find your video controller with:

$ lspci | grep VGA
01:00.0 VGA compatible controller: ATI Technologies Inc Radeon R300 ND [Radeon 9700 Pro]

Add an identifier with this string to your /etc/X11/xorg.conf

Section "Device"
	Identifier	"ATI Technologies Inc Radeon R300 ND [Radeon 9700 Pro]"
	Busid		"PCI:1:0:0"
	Driver		"radeon"
	Vendorname	"ATI"
EndSection

In your Section “Screen” add the following line:

Section "Screen"
   ...
   Monitor "foo"
   ...
EndSection

Then in the Section “Monitor”, add the same 'foo' as identifier:

Section "Monitor"
   Identifier "foo"
   HorizSync 30.0 - 81.0
   VertRefresh 56.0 - 76.0
   Option "DPMS"
EndSection

The following xorg.conf file is my current file, which is not working fine yet. Graphic Controller: Ati Radeon 9700 Pro (R300 chipset), VGA output: Nec MultiSync LCD1830, DVI output: LaCie photon20vision. Default screen is Nec MultiSync LCD1830.

# xorg.conf (X.Org 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 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 "InputDevice"
	Identifier	"Generic Keyboard"
	Driver		"kbd"
	Option		"XkbRules"	"xorg"
	Option		"XkbModel"	"pc105"
	Option		"XkbLayout"	"us"
EndSection

Section "InputDevice"
	Identifier	"Configured Mouse"
	Driver		"mouse"
	Option		"CorePointer"
EndSection

Section "Device"
	Identifier	"ATI Technologies Inc Radeon R300 ND [Radeon 9700 Pro]"
	Busid		"PCI:1:0:0"
	Driver		"radeon"
	Vendorname	"ATI"
EndSection

Section "Monitor" 
        Identifier	"MultiSync LCD 1830"
        Vendorname	"NEC"
        Modelname	"NEC Multisync 1280x1024"
        Option		"DPMS"
        HorizSync       24-80
        VertRefresh     55-85
        modeline        "1280x1024_75" 135.0 1280 1296 1440 1688 1024 1025 1028 1066 +hsync +vsync
#       Gamma           1.0
EndSection

Section "Monitor"
	Identifier	"DVI-output"
	Vendorname	"LaCie"
	Modelname	"photon20vision 1600x1200"
	Option		"DPMS"
	HorizSync	31-75
	VertRefresh	56-85
	Modeline        "1600x1200_60"  160.96  1600 1704 1880 2160  1200 1201 1204 1242  +HSync +Vsync
EndSection

Section "Screen"
	Identifier	"Default Screen"
	Monitor		"MultiSync LCD 1830"
	Device		"ATI Technologies Inc Radeon R300 ND [Radeon 9700 Pro]"
   SubSection "Display"
         Depth 24
         Virtual 2880 1200
   EndSubSection
EndSection

Section "ServerLayout"
	Identifier	"Default Layout"
	Screen		"Default Screen"
EndSection