The Radeon and SiS cards have the iDCT math stage of DVD decoding in hardware too. Thanks to this, CPU power can be VERY low - SiS back then (in 1998) claimed smooth DVD playback on 233 MHz Pentiums.
Today, when advanced sound cards eat quite a bit of CPU power for six-channel decoding and the likes, you might want some headroom ... but a Celeron-366 should cut it when given an ATi or SiS graphics card.