I built a home theater room in a basement a number of years ago and use the HTPC more as a server to record media and use as a home server to send music/movies to various places in my house. I ran Cat6 cable through out the house.
A sound card may not be needed. You definitely don't want a sound card that only uses S/PDIF due to the fact it can't send true/discreet 5.1/7.1 and beyond. If your motherboard(or video card) has HDMI out or even if it has 5.1/7.1 analog can be an option.
I use an Outlaw 975 integrated amp/surround sound processor
http://www.soundandvision.com/content/outlaw-model-975-surround-processor to hook up my HTPC, cable box and bluray player to the device using HDMI. I also like the analog inputs for older...