Add a Wired4DVD card and even a PPC601 or PPC604 can play DVDs well. It's a PCI card though, so no help for NuBus PowerMacs and certainly not for Quadras.
A DVD-RAM drive will probably work fine with an IDE to SCSI adapter like the Acard 7220U. (or is it 7720U, I always get that confused, even back when I was selling them).
Wired was selling the Wired4DVD card for $19 with free shipping as recently as 2007 but they don't appear to have it on their website any longer. Might be worth an email if one is interested.