As far as I know the original iMac did not come with a DVD drive. In order to have DVD playback you need two things. A DVD drive to read DVD discs (which are not in the same format as CD-ROMs, DVD are double-surfaced, and some are even double-sides as well ). If you dont have a DVD drive them you are cant watch DVD movies. The second thing you need is a decoded card. I think the standard ATI Rage 128 card does this.
Hope this helps
Admiral