dvi to vga, resolution too small

[max]

Problem: 
I have dual head videocard nvidia 6200 (agp) with VGA and DVI.
Resolution of monitor is 1600x1200 on VGA head. But maximum resolution
of monitor on DVI head (through DVI to VGA) is only 640x480. How to set
bigger resolution?

P.S. Sorry for my english.

My environment:
Videocard:
# lspci -k | grep VGA -A3
01:00.0 VGA compatible controller: nVidia Corporation NV44A [GeForce
6200] (rev a1) Subsystem: PC Partner Limited Device 2017
	Kernel driver in use: nvidia
	Kernel modules: nvidia

Xorg:
# Xorg -version

X.Org X Server 1.11.2
Release Date: 2011-11-04
X Protocol Version 11, Revision 0
Build Operating System: Linux 3.2.1-gentoo-r2 i686 Gentoo
Current Operating System: Linux monitoring1.korolyov.mtw.ru
3.2.1-gentoo-r2 #5 SMP Fri Feb 17 11:41:27 MSK 2012 i686 Kernel command
line: root=/dev/sda2 Build Date: 17 February 2012  09:17:16AM
 
Current version of pixman: 0.24.0
	Before reporting problems, check http://wiki.x.org
	to make sure that you have the latest version.


xorg.conf:
# cat /etc/X11/xorg.conf
# nvidia-xconfig: X configuration file generated by nvidia-xconfig
# nvidia-xconfig:  version 290.10
(buildmeister at swio-display-x86-rhel47-07.nvidia.com)  Wed Nov 16
20:32:22 PST 2011

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

Section "Files"
EndSection

Section "InputDevice"
    # generated from data in "/etc/conf.d/gpm"
    Identifier     "Mouse0"
    Driver         "mouse"
    Option         "Protocol"
    Option         "Device" "/dev/input/mice"
    Option         "Emulate3Buttons" "no"
    Option         "ZAxisMapping" "4 5"
EndSection

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

Section "Monitor"
    Identifier     "Monitor0"
    VendorName     "Unknown"
    ModelName      "Unknown"
    HorizSync       28.0 - 33.0
    VertRefresh     43.0 - 72.0
    Option         "DPMS"
EndSection

Section "Device"
    Identifier     "Device0"
    Driver         "nvidia"
    VendorName     "NVIDIA Corporation"
EndSection

Section "Screen"
    Identifier     "Screen0"
    Device         "Device0"
    Monitor        "Monitor0"
    DefaultDepth    24
    SubSection     "Display"
        Depth       24
    EndSubSection
EndSection

Resolution (DVI to VGA):
# su - username -c'export DISPLAY=":0.0"; xrandr -q'
xrandr: Failed to get size of gamma for output default
Screen 0: minimum 320 x 240, current 640 x 480, maximum 640 x 480
default connected 640x480+0+0 0mm x 0mm
   640x480        50.0* 
   320x240        51.0  

Resolution (VGA):
# su - username -c'export DISPLAY=":0.0"; xrandr -q'
xrandr: Failed to get size of gamma for output default
Screen 0: minimum 320 x 175, current 1600 x 1200, maximum 1600 x 1200
default connected 1600x1200+0+0 0mm x 0mm
   1600x1200      50.0*    51.0     52.0     53.0  
   1400x1050      54.0     55.0  
   1280x1024      56.0     57.0     58.0     59.0  
   1280x960       60.0     61.0  
   1152x864       62.0  
   1024x768       63.0     64.0     65.0     66.0     67.0     68.0  
   960x720        69.0  
   928x696        70.0  
   896x672        71.0  
   832x624        72.0  
   800x600        73.0     74.0     75.0     76.0     77.0     78.0
79.0     80.0     81.0     82.0 720x400        83.0  
   640x512        84.0     85.0     86.0  
   640x480        87.0     88.0     89.0     90.0     91.0     92.0
93.0     94.0 640x400        95.0  
   640x350        96.0  
   576x432        97.0  
   512x384        98.0     99.0    100.0    101.0    102.0  
   416x312       103.0  
   400x300       104.0    105.0    106.0    107.0    108.0  
   360x200       109.0  
   320x240       110.0    111.0    112.0    113.0  
   320x200       114.0  
   320x175       115.0