For a basic NAS you can't go wrong with an HP N54L Microserver. Put FreeNAS on top of it and you have a very capable NAS. You can run non-ECC memory on them (you could with the N40L) not advisable if you are using ZFS but can be done.
I had six 3TB 3.5" drives in my N40L
I had six 3TB 3.5" drives in my N40L
