If you're actually thinking about this, you might want to check out the Linksys NSLU2. It goes for $80. It uses external disks whch isn't as nice, but you can pick up a disk enclosure for ~$35 from newegg and choose your own drive. (250 gb drive for $220-300. Mirra's 250 server is $750 btw).
The Linksys box runs Linux and the firmware is open-source. There is a great series of articles on what you can do with this device at tom's networking
http://www.tomsnetworking.com/Sections-article85.php. (Skip to part five for pre-built firmware you don't need to compile yourself)