problem while setting refresh rate in xorg

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p.s.
This is my xorg.conf file generated by nvidia-settings:


# nvidia-settings: X configuration file generated by nvidia-settings
# nvidia-settings:  version 1.0  (root at feudaltimes)  Thu Apr 26 15:10:10 ART
2007

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

Section "Files"
    RgbPath         "/usr/X11R6/lib/X11/rgb"
    FontPath        "/usr/X11R6/lib/X11/fonts/misc/:unscaled"
    FontPath        "/usr/X11R6/lib/X11/fonts/100dpi/:unscaled"
    FontPath        "/usr/X11R6/lib/X11/fonts/75dpi/:unscaled"
    FontPath        "/usr/X11R6/lib/X11/fonts/misc/"
    FontPath        "/usr/X11R6/lib/X11/fonts/Type1/"
    FontPath        "/usr/X11R6/lib/X11/fonts/100dpi/"
    FontPath        "/usr/X11R6/lib/X11/fonts/75dpi/"
    FontPath        "/usr/X11R6/lib/X11/fonts/cyrillic/"
    FontPath        "/usr/X11R6/lib/X11/fonts/TTF/"
EndSection

Section "Module"
    Load           "dbe"
    Load           "extmod"
    Load           "type1"
    Load           "freetype"
    Load           "glx"
EndSection

Section "ServerFlags"
    Option         "Xinerama" "0"
EndSection

Section "InputDevice"
    # generated from default
    Identifier     "Mouse0"
    Driver         "mouse"
    Option         "Protocol" "PS/2"
    Option         "Device" "/dev/psm0"
    Option         "Emulate3Buttons" "no"
    Option         "ZAxisMapping" "4 5"
EndSection

Section "InputDevice"
    # generated from default
    Identifier     "Keyboard0"
    Driver         "keyboard"
EndSection

Section "Monitor"
    # HorizSync source: edid, VertRefresh source: edid
    Identifier     "Monitor0"
    VendorName     "Unknown"
    ModelName      "LG L1740B"
    HorizSync       30.0 - 83.0
    VertRefresh     56.0 - 75.0
    Option         "DPMS"
EndSection

Section "Device"
    Identifier     "Videocard0"
    Driver         "nvidia"
    VendorName     "NVIDIA Corporation"
    BoardName      "GeForce 6100"
EndSection

Section "Screen"
    Identifier     "Screen0"
    Device         "Videocard0"
    Monitor        "Monitor0"
    DefaultDepth    16
    Option         "metamodes" "1280x1024_75 +0+0; 1280x1024_75 +0+0;
1024x768 +0+0; 800x600 +0+0; 640x480 +0+0; 1280x1024_60 +0+0;"
    SubSection     "Display"
        Depth       16
        Modes      "1280x1024" "1024x768" "800x600" "640x480"
    EndSubSection
EndSection