Setting VESA display mode > 800x600 in Ubuntu 9.04

"Finally I got it. Its now working fine on 1280x1024.

I don't have to use the kernel boot parameter at all
xorg.conf is still in use, and the 'new' file monitors.xml seems to be neither written nor read by the system

The problem was a misconfiguration of my monitor. Without explicitly giving it a HorizSync the Xdotorg estimated a valid range of 28-33kHz and thus only allowed the lower modes. The logfile /var/log/Xorg.0.log told me:

(II) VESA(0): Configured Monitor: Using hsync range of 31.50-37.90 kHz
(II) VESA(0): Configured Monitor: Using vrefresh range of 50.00-70.00 Hz
(WW) VESA(0): Unable to estimate virtual size
(II) VESA(0): Not using built-in mode ""1280x1024"" (hsync out of range)
(II) VESA(0): Not using built-in mode ""1024x768"" (hsync out of range)
(II) VESA(0): Not using built-in mode ""640x400"" (hsync out of range)
(II) VESA(0): Not using built-in mode ""320x400"" (hsync out of range)
(II) VESA(0): Not using built-in mode ""320x240"" (illegal horizontal timings)
(II) VESA(0): Not using built-in mode ""320x200"" (illegal horizontal timings)

With a setting of 28-73kHz, I can now chose modes up to 1280x1024.

...
Section ""Monitor""
Identifier ""Configured Monitor""
HorizSync 28-73
EndSection
...

My screen should support even higher modes, and because I only estimated the number 73 I might have to set it even higher. However, currently I know that this is not the reason for the restriction, because modes highter than 1280x1024 are not checked at all. I think I will be able to configure those as well, and I will come back and report the solution as soon as I have it."

Investigate xrandr - at first glance it seems to be the right sort of thing.