With regards to displaying mp3s and such, the easiest way I can think of is using an iPod with iPod dock. It shows you what is playing and you can even control it with a remote. (There are several different dock options, many receivers have ones you can integrate)
I'm sure there are other mp3 players out there that would work too (zune? maybe?)
There other options like these:
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Logitech
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Sonos (a whole different price class though)
- there are other brands that make similar products
If you only want the music, without displaying them on your tv you can buy an Apple Airport express, it has a 1/8" output. You use iTunes to stream directly to the router, which you would hook up to your stereo.
With regard to HTPCs, there are also different options. They normally run XP-Media Center edition or use the Vista Media Center (or even Apple's Frontrow). A HTPCs needs to have a descent graphic card to put out HD and a tuner to be able to record tv. Theoretically you can turn any pc into a HTPC, the ones that are sold as such, just have a certain look and a really quite fan.
Hope this answers some questions....