I was given a p4 optiplex desktop which I am using for my fileserver. I have a pair of 1.5tb disks and a pair of 2tb disks in mirrors running freeNAS on a usb thumbdrive.
The optiplex is great because is quiet, fast enough and the case conveniently fits 4 drives(and free). I have a cheap 4 sata card for the drives and I splurged on a decent intel gigabit nic after couple other cheap ones with no luck (asus and dlink I believe). If you do the thumbdrive thing, make sure and buy two and dd the contents over, one failed on me and now I have to manually manage my raid via cli.
Of course this does not protect me from myself, but I haven't lost too much due to my own stupidity. I used to have a larger fileserver with all sorts of little drives, but I replaced them with a couple big ones and I'm much happier for it.
Lots of people swear by drobos. One person I know plugs a decent usb drive into his hacked router. Rsnapshot is good for automatic incremental backups.