Forwarding this bug from Ubuntu reporter Vince Kimball:
http://bugs.launchpad.net/ubuntu/+source/xserver-xorg-video-ati/+bug/542925

[Problem]
When attempting to play mpg videos on ATI ES1000, no video or audio output is seen, and dmesg shows a repeating sequence of:

[drm:radeon_cs_ioctl] *ERROR* Invalid command stream !
[ 3208.384341] Forbidden register 0x2140 in cs at 1
(..repeated)

[Original Description]
RAM allocation to the GPU cannot be controlled via BIOS.

I also tried radeon.modeset=0 and I still don't get video.

Architecture: amd64
Date: Sat Mar 20 15:35:16 2010
DistroRelease: Ubuntu 10.04
DkmsStatus: Error: [Errno 2] No such file or directory
GdmLog2: Error: command ['gksu', '-D', 'Apport', '--', 'cat', '/var/log/gdm/:0.log.2'] failed with exit code 1: cat: /var/log/gdm/:0.log.2: No such file or directory
InstallationMedia: Ubuntu 10.04 "Lucid Lynx" - Beta amd64 (20100318)
MachineType: Dell Inc. PowerEdge SC440
Package: xserver-xorg-video-ati 1:6.12.191-1ubuntu2
ProcCmdLine: BOOT_IMAGE=/boot/vmlinuz-2.6.32-16-generic root=UUID=f546d1e7-83bf-490f-82b2-ab6f1a32e5ca ro quiet splash
ProcEnviron:
 LANG=en_US.utf8
 ProcVersionSignature: Ubuntu 2.6.32-16.25-generic
SourcePackage: xserver-xorg-video-ati
Uname: Linux 2.6.32-16-generic x86_64
dmi.bios.date: 09/04/2007
dmi.bios.vendor: Dell Inc.
dmi.bios.version: 1.5.0
dmi.board.name: 0NY776
dmi.board.vendor: Dell Inc.
dmi.chassis.type: 7
dmi.chassis.vendor: Dell Inc.
dmi.modalias: dmi:bvnDellInc.:bvr1.5.0:bd09/04/2007:svnDellInc.:pnPowerEdgeSC440:pvr:rvnDellInc.:rn0NY776:rvr:cvnDellInc.:ct7:cvr:
dmi.product.name: PowerEdge SC440
dmi.sys.vendor: Dell Inc.
glxinfo: Error: [Errno 2] No such file or directory
system:
  codename:           lucid
 architecture:       x86_64
 kernel:             2.6.32-16-generic



[lspci]
05:07.0 VGA compatible controller [0300]: ATI Technologies Inc ES1000 [1002:515e] (rev 02)
    	Subsystem: Dell Device [1028:01df]