default screen resolution changes when reboot

I'm using ubuntu 10.04 and my graphics card is Nvidia Geforce 8400GS.Every time when I change my screen resolution to 1024*768 after rebooting it changes to 800*600.

xconfig status-

# nvidia-settings: X configuration file generated by nvidia-settings
# nvidia-settings: version 1.0 (buildd@palmer) Fri Apr 9 10:35:18 UTC 2010

Section "ServerLayout"
    Identifier "Layout0"
    Screen 0 "Screen0" 0 0
    InputDevice "Keyboard0" "CoreKeyboard"
    InputDevice "Mouse0" "CorePointer"
    Option "Xinerama" "0"
EndSection

Section "Files"
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 "InputDevice"

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

Section "Monitor"

    # HorizSync source: edid, VertRefresh source: edid
    Identifier "Monitor0"
    VendorName "Unknown"
    ModelName "PTW"
    HorizSync 30.0 - 58.0
    VertRefresh 50.0 - 120.0
    Option "DPMS"
EndSection

Section "Device"
    Identifier "Device0"
    Driver "nvidia"
    VendorName "NVIDIA Corporation"
    BoardName "GeForce 8400 GS"
EndSection

Section "Screen"

# Removed Option "metamodes" "nvidia-auto-select +0+0; 1024x768 +0+0"
    Identifier "Screen0"
    Device "Device0"
    Monitor "Monitor0"
    DefaultDepth 24
    Option "TwinView" "0"
    Option "TwinViewXineramaInfoOrder" "CRT-0"
    Option "metamodes" "1024x768_60 +0+0; nvidia-auto-select +0+0; 1024x768 +0+0"
    SubSection "Display"
        Modes "1024x768" "1280x960" "640x480" "800x600"
 Depth 24
    EndSubSection
EndSection