Planning a Xinerama setup is pretty straight forward. There as essentially three things you need to take into account, screen resolution, color depth, and screen layout.
You will need at least 2 graphics cards (a dual headed one should work, too) and two monitors, an operating system on which XFree runs (for instance Linux or Solaris) and XFree86 version 4.0. I assume your setup works, and that your two video cards are supported by XFree86.
Unless you recently upgraded or installed linux you are probably running an older version of X. Verify that you are running version 4.0 or better by typing the following command:
papel:/home/nico/X/bin # X -version
You should now see something like this:
XFree86 Version 4.0 / X Window System (protocol Version 11, revision 0, vendor release 6400) Release Date: 8 March 2000 If the server is older than 6-12 months, or if your card is newer than the above date, look for a newer version before reporting problems. (see http://www.XFree86.Org/FAQ) Operating System: Linux 2.3.46 i686 [ELF] Module Loader present
If the version is not 4.0 or higher ( first line ), you will need to upgrade. Use your distributions package manager to upgrade to version 4.0 or better or download it directly from XFree86 and install it.
ftp://ftp.xfree86.org/pub/XFree86/4.0/
or better use one of the mirrors found at
http://www.xfree86.org/4.0/ftp.html
After download the files install the new X with the Xinstall.sh shellscript. Please note, if you install X this way it is bypassing any package management your system has.
It is possible to have each physical screen in your Xinerama setup to have a different resolution. There are some advantages to this, I was able to use an old monitor which only operates at 640x480, and a bigger 17" at 1280x1024 in my setup. I have also heard of web developers and graphics designed who use one big "preview" screen and flank it with one or two smaller screens. I think this flexibility is one of the great things about the Xinerama extensions.
There are several UI issues which are specific to Xinerama with most current generation window managers (see the section on Window Managers ) do not address well. The most anoying is the poor handling of dead areas.
Window managers assume the display area is a rectangle equal in size to the heighth and width of your total desktop. If you use more than one display resolution in a Xinerama setup your desktop will be non-rectangular. This results in "dead areas", areas which do not exist on your display, but window manager interpret as "Empty". Many window managers will try to utilize this dead area for new windows. The result is windows which are inaccessable. As window managers become Xinerama aware and this problem will go away quickly.
Window managers also don't handle the concept of maximizing a window when you are running Xinerama. Usually what happens is it maximizes your window across all available screens. Having Netscape spread across 2 displays is generally not the best way to surf the net.
Unlike with screen resolotion, Xinerama limits your entire virtual screen to one color depth. If you were planning on pulling out a cheap video card for your second display you need to keep this in mind. If your old video card only supports 8 bit color you might get a bigger display but most newer programs look lousy in 256 colors.
Layout decisions are fairly simple, you just need to decide how you want to physically lay out your monitors. Most people will simply place their monitors in a row and view their desktop as one giant monitor. It is also possible to overlap displays, or place them in more complex layouts. Keep in mind though what I said above about window managers expecting rectangular displays.