I'm with Crazy Badger on the use of ECC RAM with ZFS (or any filesystem that's used to store data you care about), although it's certainly not required. One of the main reasons to use ZFS is data integrity and RAM is heavily used by ZFS. The data integrity features of ZFS don't really mean all that much if what's in RAM isn't correct. It's what I use in my FreeBSD/ZFS server.
While I don't use FreeNAS in my own server, I do think it's a really nice product and recommend it.