On Tue, 15 Mar 2005, Frederick Bruckman wrote:

> mga(4) says:
>
>  Support for the second head on G400 cards requires a binary-only
>  "mga_hal" module that  is  available  from Matrox
>  <http://www.matrox.com>, and may be on the CD supplied with the card.
>
> With any luck, it'll load automatically after you drop it into
> "/usr/X11R6/lib/modules/drivers/" and restart the X server, or you might
> have to generate a new XF86Config file. [I've looked into this, because I
> have an MGA550, but I've never gotten around to adding a second monitor.]

Well, I do own a 550 as well and I tried Xinerama once. It didn't 
require a special module, standard X11 supplied by NetBSD did it.
I don't exactly remember what I've done, but it wasn't more then
editing XF86Config and recompiling at least Windowmaker with
a special flag to enable Xinerama support. At that point I was
wondering, why this flag wasn't enabled by default, as Windowmaker
will dtrt at runtime.
It was fun, but I couldn't find a good reason for two monitors. :)

-- 
Tom