Lets just clear up a few things:
"When you say that some computers do not support digital audio over HDMI, do you mean some computers do not have audio on HDMI? Or do you mean that not all HDMI audio is necessarily digital audio?"
Firstly, HDMI is digital. All signals over / through HDMI is digital.
Second, Yes some HDMI ports on a motherboard or graphics card might not support AUDIO as well as the VIDEO signal via the HDMI cable.
Unfortunatly SsZERO is partially incorrect here. If anything you will be able to prove what he is saying in his statement about onboard sound being ordinary. You will notice the difference via your fairly good Amp and speaker setup between onboard generated sound and through your said sound cards.
Read many reviews of your sound cards and you will see the reviewer raving on about how good the sound is over the onboard. Although chances are that the reviewer used music rather than a movie to review the sound quality.
If I were you I would read as many reviews on your chosen sound card as possible and see what they say about using in a HTPC setup.
Hope I have helped.
Check these out:http://www.atomicmpc.com.au/Review/231426,...s-our-ears.aspx
Check out page 17 of the below:http://viewer.zmags.com/publication/ad7aab78#/ad7aab78/16