Hardware used
3 x Benq 24“ screens
1 x DiamondDigital 19” screen
2 x Nvidia Geforce 8400 GS PCI-E x16 cards

Backup your /etc/X11/xorg.conf before you start.

I found it easier if you use all the same monitors and resolution.

Put the following into your /etc/X11/xorg.conf

# nvidia-settings: X configuration file generated by nvidia-settings
# nvidia-settings:  version 1.0  (buildd@yellow)  Fri Apr  9 11:51:21 UTC 2010


Section "ServerLayout"
    Identifier     "TriHead"
    Screen      0  "Screen0" 3200 0
    Screen      1  "Screen1" 1280 0
    Screen      2  "Screen2" 5120 0
    Screen      3  "Screen3" LeftOf "Screen1"
    InputDevice    "Keyboard0" "CoreKeyboard"
    InputDevice    "Mouse0" "CorePointer"
    Option         "Xinerama" "1"    # Enable xinerama on the whole
EndSection

Section "InputDevice"

    # generated from default
    Identifier     "Keyboard0"
    Driver         "kbd"
EndSection

Section "InputDevice"

    # generated from default
    Identifier     "Mouse0"
    Driver         "mouse"
    Option         "Protocol" "auto"
    Option         "Device" "/dev/psaux"
    Option         "Emulate3Buttons" "no"
    Option         "ZAxisMapping" "4 5"
EndSection

Section "Monitor"
    Identifier     "Monitor0"
    VendorName     "Unknown"
    ModelName      "BENQ E2400HD"
    HorizSync       30.0 - 83.0
    VertRefresh     55.0 - 85.0
    Option         "DPMS"
EndSection

Section "Monitor"
    Identifier     "Monitor1"
    VendorName     "Unknown"
    ModelName      "BENQ M2400HD"
    HorizSync       30.0 - 83.0
    VertRefresh     55.0 - 85.0
    Option         "DPMS"
EndSection

Section "Monitor"
    Identifier     "Monitor2"
    VendorName     "Unknown"
    ModelName      "CRT-1"
    HorizSync       30.0 - 83.0
    VertRefresh     55.0 - 85.0
    Option         "DPMS"
EndSection

Section "Monitor"
    Identifier     "Monitor3"
    VendorName     "Unknown"
    ModelName      "NVT Novatek  563D"
    HorizSync       30.0 - 80.0
    VertRefresh     50.0 - 75.0
EndSection

Section "Device"

       # Use lscpi | grep VGA to get the BusID
                # Note the Screen numbers
    Identifier     "Device0"
    Driver         "nvidia"
    VendorName     "NVIDIA Corporation"
    BoardName      "GeForce 8400 GS"
    BusID          "PCI:3:0:0"
    Screen          0
EndSection

Section "Device"

         # A device for each screen
    Identifier     "Device1"
    Driver         "nvidia"
    VendorName     "NVIDIA Corporation"
    BoardName      "GeForce 8400 GS"
    BusID          "PCI:3:0:0"
    Screen          1
EndSection

Section "Device"
    Identifier     "Device2"
    Driver         "nvidia"
    VendorName     "NVIDIA Corporation"
    BoardName      "GeForce 8400 GS"
    BusID          "PCI:4:0:0"
    Screen          0
EndSection

Section "Device"
    Identifier     "Device3"
    Driver         "nvidia"
    VendorName     "NVIDIA Corporation"
    BoardName      "GeForce 8400 GS"
    BusID          "PCI:4:0:0"
    Screen          1
EndSection

Section "Screen"
    Identifier     "Screen0"
    Device         "Device0"
    Monitor        "Monitor0"
    DefaultDepth    24
    Option         "NoLogo" "True"
#    Option         "DoubleBuffer" "True"
    Option         "TripleBuffer" "True"
    Option         "TwinView" "0"
    Option         "metamodes" "DFP: nvidia-auto-select +0+0"
    SubSection     "Display"
        Depth       24
    EndSubSection
EndSection

Section "Screen"
    Identifier     "Screen1"
    Device         "Device1"
    Monitor        "Monitor1"
    DefaultDepth    24
    Option         "TwinView" "0"
    Option         "metamodes" "CRT: nvidia-auto-select +0+0"
    SubSection     "Display"
        Depth       24
    EndSubSection
EndSection

Section "Screen"

# Removed Option "metamodes" "nvidia-auto-select +0+0"
# Removed Option "metamodes" "1920x1080 +0+0"
    Identifier     "Screen2"
    Device         "Device2"
    Monitor        "Monitor2"
    DefaultDepth    24
    Option         "NoLogo" "True"
#    Option         "DoubleBuffer" "True"
    Option         "TripleBuffer" "True"
    Option         "TwinView" "0"
    Option         "metamodes" "CRT: 1920x1080 +0+0"
    SubSection     "Display"
        Depth       24
    EndSubSection
EndSection

Section "Screen"
    Identifier     "Screen3"
    Device         "Device3"
    Monitor        "Monitor3"
    DefaultDepth    24
    Option         "TwinView" "0"
    Option         "metamodes" "DFP: 1280x1024 +0+0"
    SubSection     "Display"
        Depth       24
    EndSubSection
EndSection