The wirelss interfaces are usefull only if you cannot run cables from your PC to the HT system. If you can run cables, just run lft and right (or digital audio) cables. You will have to get a card with audio outputs for your PC if it does not have them already. Yes the interface will be at the PC.
The new Escient Fireball line is able to not only store music on it's own hard drive, but can send and receive music from your PC. You can keep your favorites on the Fireball (with less compression and MUCH better sound quality) but also access your MP3 collection on your computer for background listening or parties. You should be able to do all of this from the FireBall interface on your TV screen AND/OR your PC.
Another option is a receiver from Integra with a ethernet jack. This plugs into your router and can receive MP3 files from your computer. Once free software is loaded into your PC, it will automatically identify audio files on your computer. It will then give you a list of these on your TV screen which you can select from via the Integra remote control. The DTR-7.4, 8.4 and 10.5 all have this feature. In addition, they are fantastic receivers.
Keep in mind that MP3 quality is TERRIBLE. You may not notice that on your PC speakers but you will hear it on when played on a good HT system. Also, the Fireball, while expensive (starting at $1999) gives you selectable levels of sound quality and offers very good compression. It also offers a very attractive, easy to use interface (with cover art!) with MANY search and playlist options. The whole family can use this interface. And it is right there on your TV screen.
