Update June'2012: I updated my systems to Linux Mint 13.0 Maya. Nouveu driver is now the only chance to use old nvidia cards (NVS 280 AGP or Quadro4 550 XGL AGP) for dual-head (two monitor display). See the my page about using the nouveaur driver with an example xorg.conf file. It still works fine as is there. Enjoy it.

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Update March'2012: It works -as is- also with my new 'old' nvidia quadro4 550 xgl  AGP !

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Setting-up a new video card or monitor is still the worst side of linux, both for beginners but also for advanced users. It is very easy to set the most common monitors in most linux systems but I have lost several hours every time I have tried to use large o special monitors.

Here I post my current working xorg.conf files that may solve most problems for users of 1080p monitors. They worked for me with old ati cards (ati rage 128 pro) and old nvidia (nvidia quadro NVS280 dual and Quadro4 550 XGL) both with the proprietary Nvidia 96 driver.

# Experts only: .in version 12.0: Install the package nvidia 96-updates and nvidia-settings-updates (Mint will do for you with Jockey ..;

In version 13.0 (Maya)  Use nouveau. You will not be offered to install any nvidia proprietary driver as it does not work nowadays (september'2012). It's a problem derived from Ubuntu 12.04 that may be fixed in the future (?).

Copy and paste my /etc/X11/xorg.conf file and it will work on most old nvidia cards. 
#

This Nvidia xorg.file is for a dual screen (xinerama) system but can be applied to any Nvidia configuration using the propietary nvidia driver. Never use nouveau (compatible free driver) if you use new kernels (> 2.6.36 - 3.0.0.x) as the Nvidia drivers is the only working solution now. They also provide much better performance .

Linux Mint 12.0: Don't use the Nouveau driver with new kernels with old Nvidia cards. Nouveau has since summer 2011 serius problems with dual-head in old cards. If you require nouveau driver (free open source driver), the modeline definition is mandatory (see part 2 of 3). I'have spent/lost 20 hours just to get the left monitor + 40% left-hand side of the second monitor working (by tweaking xrand settings).

Linux Mint 13.0: Use only the Nouveau driver with old Nvidia cards. There is no chance to use an nvidia driver (up to september 2012). Nouveau woks quite well in a dual head setup. No need for tweaking the system beside using the xorg.conf file enclosed in this post (it requires only to copy the file to "/etc/X11/" directory as sudo/root) . 

Dual Monitor - Nvidia driver - Nvidia Quadro NVS-280 AGP (also Quadro4 550 XGL) dual head AGP (VGA-DB15), dual head VGA cable, and two 1080p monitors (acer 23'' 1920x1080p, with db15 connectors) and Linux Mint Debian and Linux Mint 12 with kernels 2.6.3x up to 3.0.x

Nvidia quadro NVS 280 AGP (or quadro4 550 xgl) is a few year old model dual head video card that can be still found as a new product in internet shops (pe. as an HP OEM part). Used in CAD workstations is still a very good, low power, fan-less, high resolution 2D card for dual head applications.

Since kernel 2.6.37.x, or 3.0.0.x and up, it only works fine with the nvidia propietary driver, and should be the 96 nvidia legacy driver (version 96.43.18 and up). These new kernels provide better performace when using multiple windows and/or browser tabs.

Don't try/upgrade to newer nvidia drivers (173, 280, ... as I did...) that may deliver much better performances: they will not even detect your previously working card. You will end in a deadlock: black screen with a top-left corner flashing cursor. As usual, by booting in 'safe mode' you may execute 'dmesg | less' and you will be able to see the causes of your pains along the listing. 

Quick fix for the video deadlock: Create a short xorg.conf writting 'nouveau' or 'vesa' as the driver in place of nvidia. You may use 'nvidia-xconfig' to build the file from scratch for you, but remember to replace with nouveau. Reboot in 2.6.32 kernel and you will be able work in Gnome and use 'Synaptic' to change the drivers packages to the required version (in this case I started with legacy 96.43.18 and in Mint 12.0 with 96-updates). Edit the xorg.conf as in this tutorial, reboot, and you are done.

1.- make a copy of your xorg.file 'cp /etc/X11/xorg.conf /etc/X11/xorg.conf.my'

2.- Edit and save /etc/X11/xorg.conf (sudo gedit /etc/X11/xorg.conf) with the following content 

 # nvidia-xconfig: X configuration file compiled by ... and hand editted
 # nvidia-xconfig: taken from http://www.lx-soft.com/epia/xorg.conf

Section "ServerLayout"
Identifier "Layout0"
Screen 0 "Screen0" 0 0
InputDevice "Keyboard0" "CoreKeyboard"
InputDevice "Mouse0" "CorePointer"
EndSection

Section "Files"
RgbPath "/usr/X11R6/lib/X11/rgb"
# FontPath "/usr/share/fonts/default/Type1"
EndSection

Section "Module"
Load "dbe"
Load "extmod"
Load "type1"
Load "freetype"
Load "glx"
EndSection

Section "ServerFlags"
Option "Xinerama" "0"
EndSection

Section "InputDevice"
# generated from default
Identifier "Mouse0"
Driver "mouse"
Option "Protocol" "auto"
Option "Device" "/dev/input/mice"
Option "Emulate3Buttons" "no"
Option "ZAxisMapping" "4 5"
EndSection

Section "InputDevice"
# generated from data in "/etc/sysconfig/keyboard"
Identifier "Keyboard0"
Driver "kbd"
Option "XkbLayout" "es"
Option "XkbModel" "pc105"
EndSection

Section "Monitor"
# HorizSync source: edid, VertRefresh source: edid
Identifier "Monitor0"
VendorName "ACER"
ModelName "DFP AT2356P"
HorizSync 30.0 - 80.0
VertRefresh 60.0 - 70.0
Option "DPMS"
EndSection

Section "Device"
Identifier "Videocard0"
# Delete the line not corresponding to your current version 

# Use the following line in in linux mint 12.0 as it may use accelerated "nvidia"
Driver "nvidia" 

# Use this line in in Linux mint 13.0 as it works with "nouveau"
Driver "nouveau" 

VendorName "NVIDIA Corporation"
BoardName "Quadro NVS 280"
EndSection

Section "Screen"
Identifier "Screen0"
Device "Videocard0"
Monitor "Monitor0"
DefaultDepth 24
Option "TwinView" "1"
Option "TwinViewXineramaInfoOrder" "DFP-0"
Option "metamodes" "DFP: 1920x1080, DFP: 1920x1080 +1920+0"
SubSection "Display"
Depth 24
# Virtual 3840 1080
EndSubSection
EndSection
# end of /etc/X11/xorg.conf (xorg X Window System server configuration file) #

3.- Remember to change the keyboard layout and language/country (in blue in my setup: es = Spanish (es, es_ES or ca_ES). Fix following your needs.

In case of doubt, you may delete them and let the system apply defaults.

4.- Type [Ctrl]+[Alt]+[<- backspace] to restart your X11 graphics subsystem

5.- I hope you are done !

Working systems (two) both dual head.

1. *Dell GX-260 SFF (P4-2.66GHz, 2GB RAM) Linux Mint 12.0; small and quiet
   1. Video card nvidia low profile Quadro4 550 XGL AGP dual-head cable + cable; ebay
2. Dell GX-260 (P4-2.66GHz, 2GB RAM) Linux Mint 12.0; small and quiet
   1. Video card nvidia low profile Quadro NVS 280 AGP dual-head cable (HP oem card); ebay
3. Two Acer AT2356 1080p 23'' TFT monitors; large desktop!: I'm an old guy