It depends on how you want to use the computer.
If you want to control your music library using a media player on the computer, like WMP, iTunes, or WinAmp, then you can just connect the sound card directly to the receiver using either analog or digital cables (depending on whether your soundcard supports one or the other or both). The downside is that the computer must be pretty close to the receiver.
If you want to be able to play music stored on the computer in another room and not have to be in front of the computer to select music then you need a media server. I think the Squeezebox is the best of the bunch. If your house is wired for ethernet, you can make a hard wired connection if you have network jacks in the computer room as well as the room from which you want to listen to music. The Squeezebox can also work wirelessly if you have a wireless router. The only downside is that the computer has to be on and you have to install their server side software.