"DirectFB is a graphics library which was designed with embedded systems in mind. It offers maximum hardware accelerated performance at a minimum of resource usage and overhead." - quoted from http://www.directfb.org
I'll exclude DirectFB features from this section.
Though MPlayer is not supported as a "video provider" in DirectFB, this output driver will enable video playback through DirectFB. It will - of course - be accelerated, on my Matrox G400 DirectFB's speed was the same as XVideo.
Always try to use the newest version of DirectFB. You can use DirectFB options on the command line, using the -dfbopts option. Layer selection can be done by the subdevice method, e.g.: -vo directfb:2 (layer -1 is default: autodetect)