Problems with resolution in GX270 & Redhat

Hopefully this is right place for this topic..

I have problem with resolution. I installed Redhat Enterprise Linux 4 to my Dell Optiplex GX270 desktop. It has Intel 810 chipset (in integrated display adapter). Monitor is IBM ThinkVision L200P 20,1" TFT, which is connected via VGA cable. DVI is also available (in separated card) but it cannot be used, because L200P has only one DVI input and it is already in use by another PC. Maximum resolution which can be used is 800x600, if I try for example 1024x768 display says: "Input signal out of range".

xorg.conf says:

Section "Monitor"
Identifier "Monitor0"
VendorName "IBM"
ModelName "IBM L200P"
DisplaySize 408 306
HorizSync 31.0 - 92.0
VertRefresh 56.0 - 85.0
Option "DPMS"
EndSection

Section "Device"
Identifier "Card0"
Driver "i810"
VendorName "Intel Corp."
BoardName "Intel 865"
EndSection

Section "Screen"
Identifier "Screen0"
Device "Card0"
Monitor "Monitor0"
DefaultDepth 24
SubSection "Display"
Viewport 0 0
Depth 24
Modes "1600x1200" "1024x768" "800x600" "640x480"
EndSubSection
EndSection

In Applications -> System Settings -> Display -menu all resolutions can be found but them won't work.

In Applications-> preferences -> Screen resolution -menu can be found only 640x480 & 800x600 resolutions.

HorizSync & VertRefresh -values are correct compared to IBM's spesifications.

When changed to DVI, 1024x768 -resolution can be used and 1600x1200 is also accepted, but resolution still remains in 1024x768 though.

Any ideas?

Thanks!