You burn a Rasbian image on a SD card, boot the Pi up, get a copy of Transmission source, compile it, Run its daemon and control it via Web interface. When you finish downloading a movie, use the command 'omxplayer' to watch it.
I was told there are some 'media center distribution' who have a good interface to control the movie playback.
I'm not sure if the CPU is completely idle when using GPU via omxplayer for playback. Well, anyway, if you feel Transmission is using up too much resource, use the --enable-lightweight option when compile it.