I've been coveting a Synology 4-bay for a while. Reason: I could start with 2 2TB drives in RAID1 (the ones on my desk) and gradually upgrade to 4 4TB drives for 12TB in Synology's Hybrid RAID by simply adding & swapping disks. (Presuming Hybrid RAID works the way I think it does. Otherwise I'd just to go RAID5, but it would require the big drives up front.)
Models I'm proposing:
* DS413j - $430 retail (Amazon $380), budget model, still feature rich, gig LAN port, USB 2.0
* DS412+ - $735 retail (Amazon $621), 2.13 GHz x86-compatible Atom processor, 2 gig LAN ports, USB 3.0, support for (modest implementations of) big-boy applications (mail server, Citrix, etc., etc.).
If the drop consensus were for a 2-bay drive, I'd probably try to build a 4-bay server using one of the open source NAS distros. (But I'm very lazy in such matters.)
http://www.linuxplanet.com/linuxplanet/reports/7153/1/
I'd be upgrading from a trusty 2-bay external RAID enclosure from Monoprice, which I currently have served from a host computer. Can't afford the RS812+, but I wish these were expandable to more drives. We use the Drobo at the office and it's very good -- I've not tried to figure out it's features beyond simple storage.