I often read that 2-4GB is good enough if you disable deduplication, but I've never tried such configurations myself.

My understanding is that even beyond deduplication, ZFS performance relies heavily on caching data in memory, and without that it's actually quite slow. As in, it'll work with even 1 GB of RAM, but becomes less and less useful.

My use case would be two RAID1 mirrors plus a spare disk. I already have a 5 years old similar config on one Mini ITX Atom board with 4 Gigs RAM and Nas4Free; no problems so far. On ARM I would probably go with OMV (suggestions?) since XigmaNAS doesn't support that architecture (yet?).