This is a good time to back up your existing config file
I did it like this:
root# > cp /etc/X11/XF86Config /etc/X11/XFree86Config.working
Note: If your system may have two sets of config files XF86Config and XF86Config-4. You can either work with the XF86Config-4 files, or rename XF86Config-4 to somthing else and work with the XF86Config file. I will assume that you are working with the XF86Config file from here out. If you are working with XF86Config-4 or if your XF86Config file is located somewhere other than /etc/X11 you will need to adjust the commands accordingly.
The newer versions of X support automatic config file generation and it works very well. If this works for you you should be able to skip the next couple steps.
Put all of your video cards into your system and set up your monitors. Set everything up the way you want it when you are done, as you will have to repeat steps later if you change things.
This next step needs to be done from the console with-out X running. If you are in X, exit now. If your system uses a display manager such as xdm or gdm exit you need to stop that service.
If you need to stop a display manager from RedHat the easiest way is like this:
root# > telinit 3
for Debian or Libranet :
root# > /etc/init.d/xdm stop
If neither of these methods work you, reboot your computer and start up in single user mode.
Once you are at the console prompt and logged in as root run X with the configure option:
root# > X -configure
Once you have done this X will normally generate a configuration file called XF86Config.new in the home directory of the root user. Test out the configuration file to see if it starts X windows properly:
root# > X -xf86config /root/XF86Config.new
Chances are that your displays will come up with the generic X Windows startup screen. If so, you are golden and just need to do a little fine tuning. Exit X by pressing Control-Alt-Backspace and copy the new config file to the /etc/X11 directory:
root# > cp /root/XF86Config.new /etc/X11/XF86Config
Then continue to Section 7.3.
If your displays didn't come up properly continue to the next section to manually configure your cards.