Try using 400x240 resolution and original fps instead. It should work a lot better.

The resolution you are using now requires software scaling, so you lose both in image quality and decoding speed. Also changing fps is always a bad idea, first you get jerkiness (clearly visible on panning scenes) even if the cpu is fast enough for decoding this video, in addition you have a risk of encountering audio/video sync problems.

As for video bitrate, it should be fine with MPlayer, but you can check 'Report' after video playback to see the statistics and ensure that no frames have been dropped. Video should play nonstop till its finish on its own without you interrupting it, otherwise you will not get detailed statistics. You can use some short video clip for testing. If you get dropped frames, try reducing bitrate until it is fine. Audio bitrate for mp3 does not affect cpu usage in mplayer, choose any value you like taking only sound quality and file size into account.

Good luck.