It may just be me, but I haven't really noticed the difference between a cheap sound card and modern onboard audio. In my opinion, you'll want to spend closer to the $100 mark.
You won't be disappointed by the HT Omega Striker sound card. I've personally used it in a HTPC and it rocks. Or the Creative X-Fi Titanium cards. I'm using the Fatal1ty edition in my gaming PC.
I know it's probably not what you're looking for, but something like the sound card is definitely worth spending money on. Trust me.