Perhaps, that explains why some x264 video (1280x720p, real ones) drop some frames and lags on some "actions scenes" or "too wide, too many detailed background scenes"
there won't be gpu accelerated h264 decoding for linux in near future. even if there was support in the driver - the players couldn't use it yet.
to lower cpu usage i suggest you get the latest mplayer from svn and compile it yourself. additionally you can use "-lavdopts skiploopfiler=all" to disable h264's deblocking, this helps a lot for me.
btw: with skiploopfilter and current mplayer your sample plays fine here on my old athlonxp 2600+ with radeon 9700. ~70% cpu usage.