Firstly, why should you use Mplayer with hardware accelerated video decoding? Check out this video (look at the CPU usage for the same video when played with and without vdpau) I've just recorded:.

video

As you can see in the above video, using Mplayer without vdpau to play an HD video (H.264 - 720p used for this test), I had a 24-52% CPU usage while using Mplayer with vdpau, the CPU usage decreased to 0%! This may not be the case for everyone, but you should still see a huge improvement in playing HD videos.