The audigy cards are awesome for DVD-A. Just beware that some DVD-As have stereo 24 bit channels @ 192 kHz. In order to decode this losslessly you will not be able to use your optical/coaxial S/PDIF interface (doesn't support the bandwidth required).
Also, some X-fi cards only utilize 96kHz DACS. In this case you will not be able to decode the 24bit @ 192kHz audio.
There is a nice chart that shows the encoding structures and when they are supported here:http://en.wikipedia.org/wiki/Audio_DVD