Unless you have a 400MHz system or faster you will experience some skipping when playing back DVD's, if you have a graphics card with hardware DVD playback support you couldget by with only a 300MHz processor.
As is with only a PMMX 233MHz it will be difficult, if you happen to have an ATi Rage 128 or newer ATi graphics card you might be able to get by reasonably well though it won't be perfect. ATi has the most advanced DVD capabilities of any current graphics cards.
If you have another graphics card with hardware DVD acceleration you can try your luck... but odd's are the results won't be pleasant.
Without any form of an add-on hardware DVD decoder, or a graphics card with hardware DVD acceleration your system definitely would not be fast enough to play back DVD's capably.
Your best choice if your system can't handle it would ideally be to upgrade the processor, if that is not an option you may want to look into purchasing an add-on DVD Decoder card to off-load much of the decoding process to limit the load on your processor.
In such a case I'd personally recommend RealMagic Hollywood Plus DVD decoder, with such a card you may well be able to playback DVD's smoothly on your existing system.