My home data center is a single frankenPC that I made from an old chassis, a mainboard that has integrated Intel Atom CPU, PCI RAID card and 4 HDDs. It's running Ubuntu Server. It works well enough, Linux manages the RAID and emails me when drives are failing, it has a transmission-deamon which is a torrent client with web interface, it also runs Sickbeard that downloads tv episodes automatically for me. The box also does and stores backups of my other computers AND web servers. I have gigabit ethernet at my home and the box can saturate it (although I use WD Red drives which are not that fast).
Also, the whole setup grabs just 35W of power (it takes about 24W when I spin HDDs down, but I don't think they like it, so I don't do it).