I've been running something just like this for the past year, and setup a similar server for a friend, with great success. Just get a mini-ITX motherboard with a built-in Atom CPU, a big case, one small HDD(Possibly from an old computer, it's just for the OS), and three big HDDs(For the RAID5 containing your media files). Install Ubuntu Server 10.04, setup the HDDs in RAID5 using mdadm, and finally install the package "deluged".
"deluged" is the backend of torrent application Deluge. Just install Deluge on your other computers(It is compatible with Windows, OSX, and Linux), and connect it to the server. It can connect via te internet, and via your local LAN.
Then, to access the server, setup both SSH(For Linux and OSX boxes) and Samba(For Windows boxes).
I'd be glad to help you with the details of setting it up, since I have a lot of experience with how to best do it at this point
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