Normally these days one doesn't need to put a sound card into an HTPC as they just use digital pass-through, which is not enhanced by sound cards (not that there aren't reasons which are exceptions to the norm). Do you have a particular reason to require a sound card? If you will be passing audio through HDMI/optical/digital coax out you might be able to shave off the requirement for 2 expansion slots, which will give you a lot more options in the end.

I am also guessing that if you're planning to watch divx you know that linux doesn't support blu-ray playback, but again... always nice to make sure with this kind of project.