At this point, the FreeBSD camp would point out that they have ZFS for infinite flexibility in building multi-terabyte storage pools,
That said, both modern SSDs and multi-terabyte spinning platters are handled quite well, thank you, by FFS2 on OpenBSD
As an aside, people sometimes confuse ZFS with XFS or GlusterFS or other stuff. ZFS is designed around extreme data reliability and integrity, not huge array size or high end performance. ZFS is an all-in-one disk+filesystem that incorporates partitions, multi-parity RAID, backups, and directory structure into one unified thing. It features raid-write-hole prevention, triple-redundant checksumming of both data and metadata, built-in block duplication, advanced journaling, atomic copy-on-write, and the ability to snapshot arbitrary parts of the system which can then be rolled back after a problem, among other things. ZFS is far more than something that 'just handles multi-terabyte pools'.
Now, whether a home user NEEDS all these reliability features is a different question, but if you decide you do, OpenBSD (along with most other *nixs) doesn't have anything remotely comparable.
That said for FreeBSD and ZFS you want at least 4GB of ram anyways.
This is a common misconception. The ARC wants to cache your entire array in ram if it can, so it will expand to fill whatever's available. You can run ZFS with limited ram, you'll just see a performance hit if you try to do lots of random reads on things that aren't cached.
