No they do not. The decoder card was introduced when PC's were still underpower (as low as P-133) to play DVD movies. If you have at least a Celeron/K62 450MHz its not necessary. Now DVD decoder card do have nice features like s-video and coaxial digital audio out that not all sound/video boards offer. Of the decoder cards the Hollywood Plus is my top pick.
You could go the software decoder route with a fast enough system. PowerDVD and WinDVD are popular choices. ATI bundles a player with its boards that takes advantage of hardware-assist. I can't comment on the Xing player. Here' what I use for DVD:
Display on TV/send to VCR - Hollywood +
Display on monitor - PowerDVD 4 XP (used to use ATI but it doesn't work with Crouching Tiger, Hidden Dragon and has less options)