This is my current xorg.conf for a dual head setup with a Nvidia GPU on Arch Linux.

Section "ServerFlags"
    Option      "Xinerama"  "true"
EndSection

Section "Files"
    RgbPath     "/usr/share/X11/rgb"
    ModulePath  "/usr/lib/xorg/modules"
    FontPath    "/usr/share/fonts/misc"
    FontPath    "/usr/share/fonts/100dpi:unscaled"
    FontPath    "/usr/share/fonts/75dpi:unscaled"
    FontPath    "/usr/share/fonts/TTF"
    FontPath    "/usr/share/fonts/Type1"
EndSection

Section "Module"
    Load    "bitmap"
    Load    "dbe"
    Load    "extmod"
    Load    "freetype"
    Load    "glx"
    Load    "int10"
    Load    "record"
    Load    "vbe"
EndSection

Section "InputDevice"
    Identifier  "Generic Keyboard"
    Driver      "keyboard"
    Option      "CoreKeyboard"
    Option      "XkbRules"  "xorg"
    Option      "XkbModel"  "pc105"
    Option      "XkbLayout" "de"
    Option      "XkbVariant"    "nodeadkeys"
EndSection

Section "InputDevice"
    Identifier  "Configured Mouse"
    Driver      "mouse"
    Option      "CorePointer"
    Option      "Device"        "/dev/input/mice"
    Option      "Protocol"      "ImPS/2"
    Option      "Emulate3Buttons"   "true"
    Option      "ZAxisMapping"      "4 5"
EndSection


Section "Device"
    Identifier  "nvidia 1"
    Driver      "nvidia"
    Option      "NoLogo"        "true"
    BusId       "PCI:1:0:0"
    Screen      0
    Option      "UseEdidDpi"    "false"
    Option      "DPI"           "96 x 96"
EndSection


Section "Device"
    Identifier  "nvidia 2"
    Driver      "nvidia"
    Option      "NoLogo"        "true"
    BusId       "PCI:1:0:0"
    Screen      1
    Option      "UseEdidDpi"    "false"
    Option      "DPI"           "96 x 96"
EndSection

Section "Monitor"
    Identifier  "Monitor 1"
    HorizSync   30-98
    VertRefresh 50-180
    Option      "DPMS"
EndSection

Section "Monitor"
        Identifier      "Monitor 2"
        HorizSync       30-98
        VertRefresh     50-180
        Option          "DPMS"
EndSection


Section "Screen"
    Identifier  "Screen 1"
    Device      "nvidia 1"
    Monitor     "Monitor 1"
    DefaultDepth    24
    Option      "AddARGBGLXVisuals"         "True"
    Option      "RenderAccel"               "True"
    Option      "AllowGLXWithComposite"     "True"
    SubSection "Display"
        Depth       16
        Modes       "1280x1024" "1280x960" "1152x864" "1024x768" "800x600" "640x480"
        Virtual     0   0
    EndSubSection
    SubSection "Display"
        Depth       24
        Modes       "1280x1024" "1280x960" "1152x864" "1024x768" "800x600" "640x480"
        Virtual     0   0
    EndSubSection
EndSection

Section "Screen"
    Identifier  "Screen 2"
    Device      "nvidia 2"
    Monitor     "Monitor 2"
    DefaultDepth    24
    Option      "AddARGBGLXVisuals"         "True"
    Option      "RenderAccel"               "True"
    Option      "AllowGLXWithComposite"     "True"
    SubSection "Display"
        Depth       16
        Modes       "1280x1024" "1280x960" "1152x864" "1024x768" "800x600" "640x480"
        Virtual     0   0
    EndSubSection
    SubSection "Display"
        Depth       24
        Modes       "1280x1024" "1280x960" "1152x864" "1024x768" "800x600" "640x480"
        Virtual     0   0
    EndSubSection
EndSection

Section "ServerLayout"
    Identifier  "Default Layout"
    Screen      0   "Screen 1"
    Screen      1   "Screen 2" leftof "Screen 1"
    InputDevice "Generic Keyboard"
    InputDevice "Configured Mouse"
EndSection

Section "DRI"
    Mode    0666
EndSection

Section "Extensions"
    Option "Composite" "Enable"
EndSection