with such an old computer (heh, sorry, i know home truths can hurt!) you'll not get full quality video files to play that well. I'd recommend you used a program like
MPlayer, and then tell it to drop frames as required. my 64mb geforce card runs perfectly with any video file, but my 8mb sis 6326 card does noticably lag with video. by using
mplayer toy_story.avi -framedrop
as soon as it starts to lag, extra video scenes get thrown away. you can also do things such as reduce preprocessing before display and such to render frames faster.