The best card without a question for HTPCs is the Audiophile 2496 from midiman. This is also for playing mp3's from the computer and for spdif passthrough. Other cards resample the 44.1 bitrate of CDs to 48 which supposedly degrades sound quality.

I've been looking at this issue myself. You can get a dvd player that plays mp3 for under $200 and it's arguably more convenient to use. So why need a HTPC for basic stuff?

Lots more information at the home theater forum at avsforum

Swami