In my opinion, you should do the following:

Find somebody you know who has an old Pentium 133 laying around (or anything faster, of course). You can probably get one of these for free since somebody will have one laying around in a closet or something. Cost: nothing

Buy a ATA133 IDE controller card for the thing for about $30 which will allow you to use big disks with minimal fuss. You might also need an ethernet card.

That's it. Small, quiet, cheap, reliable. If the thing works after all these years, it will probably keep working for a very long time (failure rate of electronic devices decreases exponentially -- if its lasted this long it will probably last quite a while yet). If it doesn't, you can get another one for nothing.

Install some flavor of linux and use software raid to make a big partition. I used raid0 but I wish I had put an extra disk in and did the redundancy thing.

As someone else mentioned, hdparm in linux lets you spin down the drives.

Install Samba, and it speaks SMB.

That's it. Total cost: about $30-50 depending on whether you need an eithernet card. I used a Dell mini-tower for my first one of these. I found that a P133 could play mp3s on the console and on 3 client machines running winamp while simultaneously having new songs uploaded to it. It was plenty. I'm using another machine now, but anything like a 133 or above should be fine.

FWIW,

Jim