If you can get hold of one, a HP N54L Microserver, or the newer G7 / G8 microservers (they are all very similar).
Not 100% sure if this is a global offer, but in the UK it's £189 with £100 cashback often. I got one a couple of months back as a NAS and home media server. It's got 4 internal hot-swap SATA bays, with support for a couple more in a top-hidden bay.
It's a surprisingly powerful little machine - it's even got pci ports so you can expand it beyond its single gigabit ethernet port. People have done all sorts of upgrades on the N54L and the earlier N40L as well.
Best bang for your buck, even without the discount IMO.
I've got mine running ESXi with a Windows Homeserver share for backing up the windows PC's around the house, and a Debian vm running for Plex and my own personal time machine backups.
It fully supports RAID (hardware built in) and has space for an optional optical drive.
Mines got 8GB RAM, and will regularly be streaming 3 tv shows/movies over Plex (converting on the fly), as well as taking hourly backups from 5 computers. It never seems to struggle doing any of this at the same time.