NAS = Network Attached Storage
I set up an old Dell Dimension 8400 with Windows Home Server...all I needed to do was add a few hard drives for storage space.
Look up NAS and see if it is worthwhile to you. I mention Windows Home Server (WHS) but there are also free NAS operating systems out there
There are a lot of posts on Tom's forum regarding this topic....and your rig would be more than capable of running as a NAS