The latest weekly packages provided by mythbuntu enable you to use coreav’s binary implementation of their x264 codecs. Theoretically this means that any HD file/show that is encoded x264 can be watched with less strain on your CPU. I tried it and did not encounter much difference between the native libraries and this 15$ proprietary one. But I stumbled upon a backport of the ffmpeg libraries from the development MythTV to the 0.21-fixes branch. I tried patching the source packages from mythbuntu with this new patch but got a build error due to the rather old x264 libraries provided with Ubuntu Hardy. But using this patch I was finally able to compile it. As soon as I collect some performance data and it turns out to be worth the time I will post a how-to.
Well, now I found out that this patch crashes while watching AVI files. No-Go for me. Then again I found this repository that has a synced ffmpeg version of the Ubuntu Hardy packages available.