personally, my home server is a "Seagate Dockstar Network Adapter", a 1.2GHz ARM box that can run Debian, with some USB disks, and a USB sound card so I can stream pulseaudio stuff to my larger speakers. Speedwise, the network is the bottleneck, so having USB disks for everything works fine. I suppose if you have your stuff wired up for gigabit and really need that much more speed, going for a box with faster interfaces would make sense, though you can get ARM plug computers with SATA or eSATA, iirc.

Things it does: NAS for large file storage and backup, pulseaudio sink, transmission w/ webclient, IRC client (ssh/screen), light home webserving.

I used to have a more serious box, but I found this runs silently, very low power, does everything I need, and is tiny. Also I got it on woot for $20. I'd strongly advise a similar setup for home stuff.