I put the playback power line to be at about p2 at 500 MHz. Anything less and DVD playback required the old DVD hardware playback card. Over p2-500 MHz I can use a 12 buck (or less) ATI AGP 4 MB card.
You can try a GEEXBOX boot CD to see if that works.
I got an old PII 333Mhz from a friend and trying to fix it up for my daughter. So far, added some more memory, up to 384MB, new 120 GB hard drive, and a DVD ROM.
Playing DVDs on it is really choppy. I figure a new video card is in order instead of the on board processing. Any suggestions on a cheap PCI video card to just play DVDs on?