on my 12.04.1 ubuntu playing a .avi file in totem which contains XVID MPEG-4 uses between 50% and 70% for the process.
both cores show 50-70% cpu usage, somehow.
using mplayer cpu usage drops to 30% for the process below:

/usr/bin/X :0 -auth /var/run/lightdm/root/:0 -nolisten tcp vt7 -novtswitch -background none
since this is a amd x2 cpu isnt that too much? or does mpeg-4 need this kind of processing power?
i just installed the ati/amd driver for the gfx card via the ubuntu drivers dialog.

snippet from Xorg.0.log, looks like its running:
[    26.377] (II) fglrx(0): [uki] DRM interface version 1.0
[    26.377] (II) fglrx(0): [uki] created "fglrx" driver at busid "PCI:1:0:0"
[    26.378] (II) fglrx(0): [uki] added 8192 byte SAREA at 0x2000
[    26.378] (II) fglrx(0): [uki] mapped SAREA 0x2000 to 0xb6066000
[    26.378] (II) fglrx(0): [uki] framebuffer handle = 0x3000
[    26.378] (II) fglrx(0): [uki] added 1 reserved context for kernel
[    26.378] (II) fglrx(0): swlDriScreenInit done
[    26.378] (II) fglrx(0): Kernel Module Version Information:
[    26.378] (II) fglrx(0):     Name: fglrx
[    26.378] (II) fglrx(0):     Version: 8.96.4
[    26.378] (II) fglrx(0):     Date: Mar 12 2012
[    26.378] (II) fglrx(0):     Desc: ATI FireGL DRM kernel module
[    26.378] (II) fglrx(0): Kernel Module version matches driver.
[    26.378] (II) fglrx(0): Kernel Module Build Time Information:
[    26.378] (II) fglrx(0):     Build-Kernel UTS_RELEASE:        3.2.0-32-generic-pae
[    26.378] (II) fglrx(0):     Build-Kernel MODVERSIONS:        no
[    26.378] (II) fglrx(0):     Build-Kernel __SMP__:            yes
[    26.378] (II) fglrx(0):     Build-Kernel PAGE_SIZE:          0x1000
[    26.378] (II) fglrx(0): [uki] register handle = 0x00004000
[    26.438] (II) fglrx(0): DRI initialization successfull
but no difference in cpu usage

video information from mplayer:

mplayer data/schlaptop/Fringe\ S04\ DVDRip\ XviD\ REWARD/fringe.s04e01.dvdrip.xvid-reward.avi 
MPlayer svn r34540 (Ubuntu), built with gcc-4.6 (C) 2000-2012 MPlayer Team
mplayer: could not connect to socket
mplayer: No such file or directory
Failed to open LIRC support. You will not be able to use your remote control.

Playing data/schlaptop/Fringe S04 DVDRip XviD REWARD/fringe.s04e01.dvdrip.xvid-reward.avi.
libavformat version 53.21.0 (external)
Mismatching header version 53.19.0
AVI file format detected.
[aviheader] Video stream found, -vid 0
[aviheader] Audio stream found, -aid 1
VIDEO:  [XVID]  640x352  12bpp  23.976 fps  982.3 kbps (119.9 kbyte/s)
Clip info:
 Software: Nandub v1.0rc2
Load subtitles in data/schlaptop/Fringe S04 DVDRip XviD REWARD/
Failed to open VDPAU backend libvdpau_nvidia.so: cannot open shared object file: No such file or directory
[vdpau] Error when calling vdp_device_create_x11: 1
[VO_XV] Could not grab port 143.
Opening video decoder: [ffmpeg] FFmpeg's libavcodec codec family
libavcodec version 53.35.0 (external)
Mismatching header version 53.32.2
Unsupported PixelFormat 61
Unsupported PixelFormat 53
Unsupported PixelFormat 81
Selected video codec: [ffodivx] vfm: ffmpeg (FFmpeg MPEG-4)
Requested audio codec family [mpg123] (afm=mpg123) not available.
Enable it at compilation.
Opening audio decoder: [ffmpeg] FFmpeg/libavcodec audio decoders
[mp3float @ 0xb639cea0]Header missing
AUDIO: 48000 Hz, 2 ch, floatle, 130.5 kbit/4.25% (ratio: 16314->384000)
Selected audio codec: [ffmp3float] afm: ffmpeg (FFmpeg MPEG layer-3 audio)