Whoops -- missed this when it was new/current:
It's been years and years since I did this but I ran DVDs on a G3/Pismo back in the day, on those and newer the Rage128 could/should have helped with DVD decoding if i remember right. In addition, I don't remember my Pismo being stuttery with DVD playback, in either OS X or OS 9. (Though: I almost never multi-tasked DVD watching at that time, so maybe I don't remember it happening because I didn't create a scenario where it would. Classic Mac OS is bad at multi-tasking, if this is exclusively a multi-tasking thing, that may well just be the answer.)
It surprises me a bit to hear that a G4 is having problems with DVD playback and I'm tempted to say it might be something other than the DVD software itself -- e.g. 20-year-old thermal interface material is worn out and the CPU is clocking down, or these machines actually relied a fair bit on their batteries and the CPU is clocking down, or the optical drive or disk has something wrong, etc etc.
Alternately: I wonder if there's some extension that's missing. (But again if the symptom happens only when multi-tasking...)
In theory, AltiVec or the Rage/Radeon should accelerate DVD playback enough in similar enough ways to the PC card decoder, I don't know that I think it'd make a difference in multi-tasking. I suppose the worst you can do is spend lots of money separating something meant exclusively for one generation of PowerBook from a machine it'd work with only to find out it doesn't do what you hoped it would.