what software cocktail did you use for playback today? I read that either the divx.com suite or the 3ivx.com "Divx Doctor" is better optimized for G4's, and the other one is better for G3's. And I suspect that the ffmpeg core of VLC and Divosx may have the lowest playback requirements of all. Anyways, if one doesn't work, try another.
I also have a pismo 400 with 576 MB ram and I've never had a problem playing divx, unless it had ac3 or MS GSM audio or something