You should configure each of your video cards to work properly with a single X server, which is actually beyond the scope of this document. You should refer to the documentation that came with your distribution, but some general hints couldn't hurt.
The easiest way would be to use the same kind of monitors & video cards, you could then configure only the first card/monitor pair, make copies of this configuration file for the number of video cards you have, and then only adjust the BusID "PCI:x:xx:x" field in the configuration file. You can do this with the help of lspci, XFree86 -scanpci -verbose , or other similar distribution-specific tools.
You could use a similar approach if you have only monitors or video cards of the same type.
Most modern distributions also have advanced tools for easier configuration of Xinerama. You can use these tools to set up the system for Xinerama and then use this configuration file for generating the configuration files for the different X servers. You can use an example configuration file, replacing video card and monitor section, by the corresponding sections from the Xinerama XFConfig-4 file.
Other useful resources: