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
Before we start the multihead portion of this process you need to have ALL of your existing cards working properly with the display they will have in the final configuration. If you haven't already, configure and install each different video card/ monitor combination you are going to have in your final setup. If you have several identical video cards you can get away with configuring one and copying the configuration for the other cards.
After you have each card set up, back up or print it's config file as you will need it later. Here's how I did it:
root# > cp /etc/X11/XF86Config /etc/X11/XFree86Config.Matrox
If your video cards are identical you can probably get away with just one copy. However don't skip this step, If all else fails this will be you backup config file incase my instructions lead you astray.
Once you have all of your displays configured you are almost there...