I have just finished massively cleaning up this page. There was lots of opinion, and lots of anecdotal information that was disorganized and unhelpful. I've added the new "HD Playback Reports" page with a script to generate useful output for HD playback. Please run the script on HD files that work smoothly for you and help me expand that page. I think it will be cleaner and more helpful than inserting stuff like "you need at least <randomprocessornumber> to play back 1080p." Thanks!
To whomever inserted the bit about how the (sic) "x264 codec" being more difficult to play back than the AVC found on Blu ray disks, x264 is not a codec. It's an encoding program. h.264 is a codec. Also, h.264 and AVC are the same exact thing.
Moreover, a P4 might manage Bluray playback on windows, where it can take advantage of hardware offload, but that's not going to work under linux. The reason that downloaded stuff is occasionally difficult to play is because of people utilizing encoder options that they don't understand that give negligible or nonexistent quality improvements and major expense of speed. Also, most raw blu-ray rips are multi-slice encoded and can be multithreaded under linux, whereas some pirates don't know/care about slicing and so don't use it.