Raid enclosure question

I am in the market for a new back-up drive which I would like to use to back-up my SSD (240GB) and my internal hard drive (2TB) where I keep my iTunes collection. I want this back-up to still be usable when I eventually replace my iMac which as it is a mid-2010 has no thunderbolt port.
I have found that the thunderbolt drives have no other ports (FireWire etc) so I've worked out that the best solution is to get a RAID array which has FireWire 800 and USB 3 (which I can eventually use with my new iMac).
I've narrowed it down to three options
1: 4 TB NON-RAID G-TECH

http://store.apple.com/uk/product/H...ive-professional-external-hard-drive?fnode=5f

2: 6TB RAID Lacie

http://www.amazon.co.uk/LaCie-2big-...UTF8&colid=XOFBTXKTWLN2&coliid=I2ICQHDHFXU0A5

Or 3: Icybox RAID Enclosure (4 Bay) with TWO 3 TB Barracuda drives

http://www.amazon.co.uk/Seagate-inc...UTF8&colid=XOFBTXKTWLN2&coliid=I3AN6JNTTV9Y2Y

http://www.amazon.co.uk/Icybox-Exte...UTF8&colid=XOFBTXKTWLN2&coliid=I1Z8QWPWEN1RDQ

The last option (3) is probably my preferred option as it is expandable BUT will it work with only 2 hard drives in a four drive enclosure? How much trouble is it to set up? Would I in fact be better off in the long run if I bought option 1 (4TB single drive) then replaced it when necessary (when/after buying new mac) with a Thunderbolt RAID array?

Thanks in advance.
Your gratefully,
John