It's my xorg.conf, adapts it to your requirements:

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# /etc/X11/xorg.conf (xorg X Window System server configuration file)
#
# This file was generated by dexconf, the Debian X Configuration tool, using
# values from the debconf database.
#
# Edit this file with caution, and see the /etc/X11/xorg.conf manual page.
# (Type "man /etc/X11/xorg.conf" at the shell prompt.)
#
# This file is automatically updated on xserver-xorg package upgrades *only*
# if it has not been modified since the last upgrade of the xserver-xorg
# package.
#
# If you have edited this file but would like it to be automatically updated
# again, run the following command:
#   sudo dpkg-reconfigure -phigh xserver-xorg

Section "Files"
        # path to defoma fonts
    FontPath    "/usr/share/X11/fonts/misc"
    FontPath    "/usr/share/X11/fonts/100dpi:unscaled"
    FontPath    "/usr/share/X11/fonts/75dpi:unscaled"
    FontPath    "/usr/share/X11/fonts/Type1"
    FontPath    "/var/lib/defoma/x-ttcidfont-conf.d/dirs/TrueType"
    FontPath    "/usr/local/share/fonts"
EndSection

Section "Module"
        Load    "GLcore"
        Load    "i2c"
        Load    "bitmap"
        Load    "ddc"
        Load    "dri"
        Load    "extmod"
        Load    "freetype"
        Load    "glx"
        Load    "int10"
        Load    "type1"
        Load    "vbe"
EndSection

Section "InputDevice"
        Identifier      "Generic Keyboard"
        Driver          "kbd"
        Option          "CoreKeyboard"
        Option          "XkbRules"      "xorg"
        Option          "XkbModel"      "pc105"
        Option          "XkbLayout"     "it"
EndSection

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

Section "Device"
        Identifier      "ATI Technologies, Inc. Radeon Mobility X700 (RV410 PCIE)"
        Driver          "fglrx"
        BusID           "PCI:1:0:0"
EndSection

Section "Monitor"
        Identifier      "Monitor Generico"
        Option          "DPMS"
        HorizSync       28-84
        VertRefresh     43-60
        Option          "MonitorLayout"   "LVDS,AUTO"
EndSection

Section "Screen"
        Identifier      "Default Screen"
        Device          "ATI Technologies, Inc. Radeon Mobility X700 (RV410 PCIE)"
        Monitor         "Monitor Generico"
        DefaultDepth    24
        SubSection "Display"
                Depth           1
                Modes           "1680x1050"
        EndSubSection
        SubSection "Display"
                Depth           4
                Modes           "1680x1050"
        EndSubSection
        SubSection "Display"
                Depth           8
                Modes           "1680x1050"
        EndSubSection
        SubSection "Display"
                Depth           15
                Modes           "1680x1050"
        EndSubSection
        SubSection "Display"
                Depth           16
                Modes           "1680x1050"
        EndSubSection
        SubSection "Display"
                Depth           24
                Modes           "1680x1050"
        EndSubSection
EndSection

Section "ServerLayout"
        Identifier      "Default Layout"
        Screen          "Default Screen"
        InputDevice     "Generic Keyboard"
        InputDevice     "Configured Mouse"
EndSection

Section "DRI"
        Mode    0666
EndSection

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GP