Sadly, PCs still just don't compare to a good DVD home theater system. $200 stereo + $200 DVD player + $300 TV = excellent $700 home theater, without the hassle of trying to make your computer decode 5.1 audio (if only we had $700 to throw around).
My understanding is that audio cards don't decode DVD movies, for the simple reason that if they did, there would have to be some coordination between the audio driver and the video driver, to ensure that the audio playback stays in sync with the video playback. That's a hard trick to pull off. As it stands now, the DVD playback software accesses special features of the video driver to use video hardware for decoding image information. It would be nice if audio cards had some Dolby 5.1 specialized hardware, and drivers that support PowerDVD in accessing these devices to unload the audio decoding from the CPU. Of course, I suppose a lot of things would be nice; all we can do is look to the future and hope that Creative and other sound card manufacturers will see the consumer desire for this feature.
One thing I did want to mention is that some mainboards are supporting 6-channel stereo output through the built-in AC 97 audio. The board itself has an SP-DIF connector for this purpose. Even power users are beginning to look upon built-in audio as a legitimate solution, as the only real advantage of a PCI audio card is 3D positional sound (which is used by only a few applications, and can be integrated into AC 97 audio if the manufacturer so desires). Sadly, audio cards just aren't the big business that graphics cards are, and thus there isn't a lot of money to drive advancements (such as hardware Dolby decoding).
<A HREF="http://bible.gospelcom.net/cgi-bin/bible?passage=PS+17:7-9" target="_new"> PS 17 </A>