720p h264 .mp4 and mkv's plays very well on the MacMini 1.5Ghz, directly from USB stick.
I'm really impressed with how well mplayer works on this machine.

For comparisation, the same movies is stuttering and gives picture distortion when I try them under OSX and mplayer (same with VLC or quicktime). I also have a PC nearby with 1.6Ghz P4, XP and 512MB ram, and a radeon 9800, and even there the same movies have trouble playing.

On MorphOS the 720p files shows high CPU usage, but this doesnt reflect on mplayer. I can move the movie window around, jump quickly back and forward in the movie without anything stuttering.
Trying to jump forward or backward on OSX or on the mentioned PC makes the movie freeze for some seconds, and usually picture problems when resuming.