After further searching I came across NASLite. It's a small, efficient, Network Attached Storage (NAS) Server Operating System designed to transform a basic computer into a dedicated SMB/CIFS, NFS, FTP and HTTP file server.
They offer it in CDrom and USB Booting, so you can boot from a small USB stick, the thing runs entirely in RAM, and you can use all your IDES for HDDs.
Anyone used this? Seems like it's got everything I need.