Para configurar la tarjeta nvidia con debian testing, añadir contrib-nonfree.

# apt-get install nvidia-kernel-common module-assistant
# apt-get install nvidia-kernel-common module-assistant
# m-a -i prepare
# m-a a-i -i -t -f nvidia-kernel
# depmod -a
# apt-get install nvidia-glx nvidia-glx-dev xserver-xorg
Este último creo que no hace nada
# dpkg-reconfigure xserver-xorg

Mi xorg.conf para los 4 monitores:

Section "Device"
        Identifier      "UNO"
        BoardName       "nVidia Corporation G72 [GeForce 7300 SE]"
        Driver          "nvidia"
        BusID           "PCI:5:0:0"
        Screen 0
EndSection


Section "Device"
        Identifier      "DOS"
        BoardName       "nVidia Corporation G72 [GeForce 7300 SE]"
        Driver          "nvidia"
        BusID           "PCI:5:0:0"
        Screen 1
EndSection

Section "Device"
        Identifier      "TRES"
        BoardName       "nVidia Corporation GeForce 8600 GT"
        Driver          "nvidia"
        BusID           "PCI:4:0:0"
        Screen 0
EndSection

Section "Device"
        Identifier      "CUATRO"
        BoardName       "nVidia Corporation GeForce 8600 GT"
        Driver          "nvidia"
        BusID           "PCI:4:0:0"
        Screen 1
EndSection

Section "Monitor"
        Identifier      "Monitor uno"
        Option          "DPMS"
        HorizSync       28-64
        VertRefresh     43-60
EndSection

Section "Monitor"
        Identifier      "Monitor dos"
        Option          "DPMS"
        HorizSync       28-64
        VertRefresh     43-60
EndSection

Section "Monitor"
        Identifier      "Monitor tres"
        Option          "DPMS"
        HorizSync       28-64
        VertRefresh     43-60
EndSection

Section "Monitor"
        Identifier      "Monitor cuatro"
        Option          "DPMS"
        HorizSync       28-64
        VertRefresh     43-60
EndSection

Section "Screen"
        Identifier      "UNO"
        Device          "UNO"
        Monitor         "Monitor uno"
        DefaultDepth    24
        SubSection "Display"
                Modes           "1280x1024"  "1024x768" "800x600" "640x480"
        EndSubSection
EndSection

Section "Screen"
        Identifier      "DOS"
        Device          "DOS"
        Monitor         "Monitor dos"
        DefaultDepth    24
        SubSection "Display"
                Modes           "1280x1024" "1024x768" "800x600" "640x480"
        EndSubSection
EndSection


Section "Screen"
        Identifier      "TRES"
        Device          "TRES"
        Monitor         "Monitor tres"
        DefaultDepth    24
        SubSection "Display"
                Modes           "1280x1024" "1024x768" "800x600" "640x480"
        EndSubSection
EndSection

Section "Screen"
        Identifier      "CUATRO"
        Device          "CUATRO"
        Monitor         "Monitor cuatro"
        DefaultDepth    24
        SubSection "Display"
                Modes           "1280x1024" "1024x768" "800x600" "640x480"
        EndSubSection
EndSection


Section "ServerLayout"
        Identifier      "Dos pantallas"
        Option "Xinerama" "on"
        Screen "TRES"
        Screen "CUATRO" RightOf "TRES"
        Screen "UNO" RightOf "CUATRO"
        Screen "DOS" RightOf "UNO"
        InputDevice     "Generic Keyboard"
        InputDevice     "Configured Mouse"
EndSection