first you have to have some sort of audio video card. I recommend the ATI Xclaim VR 128 w TV tuner (its a mac version of the xclaim vr all-in-one).
Its a pretty good card (www.epinions.com if you dont believe me). it has a tv tuner, video in (3 rca and s-video) and video out (s-video, tv-out and 1 rca). its a pretty good gaming card (its the exact same one that comes with the g4, only this one is more multimedia).
basically, you put in the dvd, put the screen to full screen mode, and use the tv-out (s-video) or video rca to plug into the appropriate port on your tv. then for sound, if you have a sweet set of compu-speakers (ie Cambridge Soundworks Digital FPS2000) the you are fine. Else you can buy a mini-plug-->rca audio converter from radio shack and pipe sound into your stereo system.
i think ati makes the best card for mac. the other companies (formac, voodoo) suck IMHO.
there is also the ITV VIew and such products from the mac catalog people. basically its just a fancy vga-->s-video converter. It runs about 150, but the ati card does tons more for 177 @
www.buy.com