By the time you buy a case, ram & HDs why not just buy a NAS HD
In what sense? A proper RAID NAS box (like the QNAP or Synergy stuff) is going to be a lot more expensive and has no option for adding extra drives: Once you're done, you're done. With something like this I can always add drives of differing sizes whenever I like and they don't have to match, although there is a clear performance hit on that. A £1500 solution becomes a lot more palatable if I can start with a £200 box and add stuff on rather than get hit with the full whack in one go
Case I already have along with PSU, RAM I might be able to steal out of my old PC as well if it's compatible, and then it's just the drives themselves. Oh, and the £80 or so for UnRAID.