How to get 1280x1024 out of Radeon RV100 on Ubuntu 7.04 ??

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I've just installed Ubuntu 7.04 on an Athlon machine with Radeon RV100 graphics. The display adapter wasn't auto-detected, and I was initially only offered two screen resolutions: 800x600 and 640x480. I want to run at 1280x1024 at 60Hz, which is supported by my monitor.

Following some instructions that I found elsewhere I edited /etc/X11/xorg.conf and added 1280x1024 and 1024x768 to the "Screen" section. This expanded my range of available screen resolutions to include 1024x768, but 1280x1024 is still not available.

My next thought was that maybe I needed to manually install a driver for the RV100, but I can't find one anywhere. Can anyone advise me on how I can get 1280x1024 resolution out of this adapter?

The Monitor and screen sections from my /etc/X11/xorg.conf file are below:

Section "Monitor"
Identifier "Generic Monitor"
Option "DPMS"
HorizSync 28-51
VertRefresh 43-60
# 1280x1024 @ 60.00 Hz (GTF) hsync: 63.60 kHz; pclk: 108.88 MHz
Modeline "1280x1024_60.00" 108.88 1280 1360 1496 1712 1024 1025 1028 1060 -HSync +Vsync

EndSection

Section "Screen"
Identifier "Default Screen"
Device "Generic Video Card"
Monitor "Generic Monitor"
DefaultDepth 24
SubSection "Display"
Depth 1
Modes "1280x1024" "1024x768" "800x600" "640x480"
EndSubSection
SubSection "Display"
Depth 4
Modes "1280x1024" "1024x768" "800x600" "640x480"
EndSubSection
SubSection "Display"
Depth 8
Modes "1280x1024" "1024x768" "800x600" "640x480"
EndSubSection
SubSection "Display"
Depth 15
Modes "1280x1024" "1024x768" "800x600" "640x480"
EndSubSection
SubSection "Display"
Depth 16
Modes "1280x1024" "1024x768" "800x600" "640x480"
EndSubSection
SubSection "Display"
Depth 24
Modes "1280x1024" "1024x768" "800x600" "640x480"
EndSubSection
EndSection