The CPU requirements for playing HDTV are significant. While XvMC support (NVIDIA only) offloads some of the work to the video card, it is not recommended to design a new system around using XvMC since it can be a hassle to get working well.
The actual minimum requirements are hard to pin down, as people have reported conflicting results. Relevant factors may include the bus speed, memory speed, CPU speed, kernel version, Linux/BSD distribution, and compiler version and optimizations.
* Use two RAM DIMM's to allow for dual channel operation which can provide a significant boost in overall system speed.
1080p+
For more information on ripping and playing Blu-ray and HD-DVD disk formats, see High Definition Disk Formats.
Many users who believe their machine capable of HD-DVD or Blu-ray playback have only attempted to play transcoded rips which, as discussed above, often have their bitrates reduced by 50 to 75 percent.
Blu-ray and HD-DVD rips are, for feature only, generally between 17 GB and 35 GB, and up to 50 GB for the entire disc. The bitrate generally varies between approximately 20 Mbit and 40 Mbit. Most Blu-ray and HD-DVD discs now use E-AC3, mlp, or Dolby TrueHD as their only sound format. A few older discs have both the new formats and AC3 Tracks, but this is less and less common. Ffmpeg SVN was recently updated to support TrueHD/mlp. Support should appear soon in mplayer. E-AC3 patches are in line for inclusion in ffmpeg, and are available on the ffmpeg development list. The only way to play back the audio on new Blu-ray and HD-DVD rips currently is to patch mplayer.
Some side Notes...
HDTV recordings typically consume slightly under 9GB per hour.
As of 0.19 a "lossless" mpeg2 transcode has been implemented. This method is used for the default transcode. This method will allow the user to cut commercials without losing any of the HDTV quality as only the frames necesary to be reencoded to maintain proper playback will be encoded. For commercial cutting these frames are usually all black, with a possible station logo, and so no quality loss should be seen at all. This transcode also has the added benefit of audio sync correction similar to what ProjectX does, as well as converting the MPEG2 TS stream to a PS stream with up to a possible 20% savings in file size on top of any cuts made.
Hope this answers your question
Reference: Wikipedia/HD/1080p/Personal Experience