These sites indicate that Linux is one of the supported OS's for the P650,
though the Matrox site does not:

http://computing.kelkoo.co.uk/b/a/ps_508827/111601.html
http://www.novatech.co.uk/novatech/specpage.html?MAT-P6564D
http://www.pcsforeveryone.com/itemdesc2.cfm?type=p&itmID=1191663&ItmS...
http://www.newegg.com/app/viewproductdesc.asp?description=14-106-153&...

This article indicates it runs, though with display artifacts, using a 8-month
old driver:

http://sourceforge.net/mailarchive/forum.php?thread_id=4120493&forum_...

This guy has nothing nice to say about Matrox:

http://andrew.hanenkamp.com/index.cgi/2004/08/04

   "Okay, I'm afraid I'm going to have to do this again. Everyone, if you use
   Linux and want to use dual monitors, do not purchase a Matrox card. Matrox's
   idea of a Linux driver is to take the contents of a dirty toilet and proceed
   to build code from those contents. A disgusting image, but I don't want
   anyone to doubt the sincerity or strength of my conviction that Matrox and
   dual-head Linux do not belong in the same document---and yet, here they are.
   My particular experience is with the Matrox P650, don't do it. If you want a
   headache, there are lots of ways to get one that are much more fun in the
   meantime.

   Anyway, I highly recommend NVidia or ATI if you want to do Linux and a
   dual-head display."

This guy laughs at the puniness of this graphics card:

http://www.micromart.co.uk/content/reviews/default.asp?Category=artic...

   "With only 64MB of dedicated RAM and 3D capabilities that would make a
   similarly priced nVidia card laugh until its lungs bleed, the Parhelia-LX
   based Matrox P650 appears expensive. When it comes to pure pixel-pushing
   power the Matrox P650 wouldn't even frighten a Playstation, but it is not to
   the geeky gamer that this card is marketed."

I like this one:

http://www.darksmile.net/hardware.html

   "Windows Based Systems

       maximus - SuperMicro P3TDDE Dual 1.2GHz, 1GB Memory. 250GB total disk
       space. Matrox P650 Dual head Video Card. This is my main system for
       MPLAB programming, and simulation/Engineering programs. It runs ok, but
       with WIN2K, I have to reboot often."

Sounds to me like this card is a bit of a dog, and everyone but Matrox is
reluctant to support it, and they did a half-assed job.  A self-fulfilling
prophecy of failure.