Since you're using MPlayer, you'd want to add the "-vo xvmc" switch to the MPlayer command line. As you haven't supplied any details on how you're launching mplayer etc, I can't give any more details than that. Browse the web, I'm sure you'll find what you're after. http://gentoo-wiki.com/HOWTO_MPlayer
is a nice, brief introduction.
Ask back here if you can't find what you need, and give us details about your system, Linux distro, how you're launching MPlayer etc.