Thanks for the suggestions! I called both Synology and QNap and they will support 3TB drives very soon. So I'll probably wait for that; otherwise, I would probably have to go with an 8-bay system to make sure we have room to expand.
The two systems I'm considering are:
1. Synology 5 bay with the 5 bay expansion (with RAID-5): http://www.synology.com/us/products/DS1010+/index.php
2. Qnap 6 (or 8) bay (with RAID-5): http://www.qnap.com/pro_detail_feature.asp?p_id=145
Someone else recommended a Netgear Pro 6-bay.
As for the back-ups, I'm thinking of using the Synology 5 bay expansion as a cost-effective back-up for the Synology 5 bay NAS. Or, if I go with Qnap, then buying another 6-bay Qnap.
Alternately, I'm considering only backing up some of the data, and buying a smaller, 2-bay NAS, which I can store in a location outside of the office when not performing back-ups.
Any other suggestions for backing up the NAS?