I don't know about sound quality and the soundcard part of the question, but for the overall experience this is a good case for Windows Media Center. You could setup a PC connected to your television and sound system and use a remote control to browse your library on the TV and listen to it through your existing sound system.
For that I'd recommend a good video card in addition to a good sound card, as this setup really makes the most sense if you use the media center for TV, DVD etc.
You could also use a media center extender and have the computer in a different room, and for that you could even use an xBox 360 as it functions as a Media Center extender.
There are a lot of other solutions out there to let you play networked media on your home entertainment system, but this is the one I recommend as it offers a lot of flexibility and the Windows-based system makes it a nice bridge between media from your computer and your living room.