do you want to watch a HD film on your Ubuntu Box? Most likely this will be a choppy experience unless you have a NVidia card with VDPAU decoding support. With ordinary open source radeon or intel drivers, your CPU wont get any help with decoding and thus the decoding will be most likely be not fast enough.
But the good news is that you dont have to wait until next christmas, when the Gallium based open source drivers will probably be able to offer video decoding acceleration. In case you have a multi-core system you probably dont need any GPU based acceleration at all, since your CPU is basically fast enough, but is just not programmed properly. Currently ffmpeg(the library which does the decoding) only uses one core of your quad-core CPU ideling the other 3 while your video is stuttering.
Luckily there is a branch of ffmpeg which uses all 4 cores called ffmpeg-mt and there is also a special version of mplayer(mplayer-mt) available in this PPA which uses this ffmpeg branch. If you use this you most likely will be able to watch HD movies now – but the proper fix would be patching the system ffmpeg library so totem, VLC et. al. would benefit from it as well…