If you want 7.1 you'll need a sound card that has 7.1 channels out. M-Audio used to make one, but I no longer see it as a product, but here's an old link:http://www.m-audio.com/products/en_us/Revolution71-main.html

5.1 of course will a lot easier because you can use coaxil or spdif and there are many mother boards that will have it.

I'd checkout some cards on zzounds.com or newegg.com.

What HTPC software are you using?