"Simple and upgradable" sounds like what your wife is suggesting ... eSATA USB drives. Especially if you don't need the "real time" protection the RAID solutions provide. If you are comfortable with nightly backups, they could probably serve you pretty well.
The other option would be to put together an inexpensive Windows Home Server machine. It doesn't need a cutting edge processor, graphics card, etc. WHS comes with some pretty good backup software and has a media server included. HP is packaging a 500gb version for less than $600 (see http://www.amazon.com/o/ASIN/B000UY1WSK/homeserver-20
for an example). The HP version includes 4 empty drive bays for expansion.
If you have an older machine laying around, it might serve this purpose as well.