Zfs
I would recommend ZFS. It is from SUN and is an enterprise-level filesystem. You can have RAIDZ which is essentially RAID 5, plus snapshots, plus sharing over iSCSI, NFS and CIFS. You can also add drives of different sizes to grow the NAS later. You can details at...
SS-4200 with ZFS
I know that you are looking at ReadyNAS but if you are ready for some tinkering, you can try Intel SS-4200 and install ZFS. I have 4 1TB SATA drives and data shared over NFS, CIFS and iSCSI plus you get the benefits of snapshots with ZFS - all for under $500. You can view the...
FreeNAS w/ZFS on SS4200
I have setup a detailed instructions on setting up FreeNAS with ZFS on my ZZ4200. http://bitsforbytes.blogspot.com/
Rock solid performance an flexibility.