Wow, thats a cute little Proliant. It's not using a storage controller I recognize as a SmartArray, so I can't comment on the reliability. Specs only list Raid 0 and 1, so at most you will have 2 TB worth of usable space with your 4x1TB disks. You could always do software raid 5 though to get 3TB.

With no OS out of the box, it would be just like turning any normal PC into a NAS. You can look at options ranging from Windows Home Server (not recommended since MS can't seem to implement and keep a drive expanding strategy), and unix based ones like FreeNAS.