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