[katnapz]

Yes you're right...it's a mini-pc. You probably googled the term "mini itx?" Check out www.mini-itx.com as it's one of the top links. Plenty of ideas and overview. On the right hand side you'll see links to projects that have been done up by others. People have figured out how to fit the things into just about anything (toasters/stuffed bears/you name it). Keep going through the google search and you'll find other info.

Building a system based on a mini board is pretty much like a regular computer. You have to keep any eye on the specs to make sure that whatever you get is compatible with what memory and such. For the most part,...most of the regular components you'd put in a full size system will work. Depending on the board though, it may not take a regular power supply (the connnector takes up too much room) so you might have to buy what's considered an "external" power supply that basically has a small 12v plug at the end. If you stick with one of the lower end (meaning slower) boards, you can use the system fanless. So unless you go brute force the system can be pretty much silent.

There are also micro-itx boards that are a bit larger and I don't believe any of those are made with the CPU built in. I've used these systems for awhile now made by a company called Shuttle. Mainly because it's just been easier to get the barebone system straight out (ok, I've gotten a bit lazy). Great for home theater use. My first one had a 900 Pentium 3, one gig hard drive (you don't need a large drive when you're streaming everything from the other computers around the house), 256 megs of ram and Win98. Minimal Windows install still left me with about 500 megs of hard drive space. It played any music and movie I wanted. You really don't need a top of the line system for simple stuff like that. If I wouldn't have had a weird electrical accident with the system I'd probably still be using it for home theater, but I've moved on to a Shuttle Zen.

I haven't bought any boards for awhile now so can't give you any pointers to sites with the lowest prices. I'd probably just look for someone clearing out the older/slower ones on ebay if I was looking though. Like I said, it doesn't take much to play music. If you've got spare old parts sitting around there's a good chance you can use them to save money. You may be confusing RAM with hard drive space from your comment above. You'll be storing your mp3s in a hard drive just like a regular computer, but...like I mentioned above, if you network the system you can just stream everything from other computers on your network.
Hope this helps you on your way.