Brainstorm: server and backup

My file server at home has 2TB (accessible) in a RAID 5 configuration (which has already saved me once). At this moment I only have 370GB free, and have already started thinking about how to grow it properly. To back up my server, I currently use a 1.6TB Infrant ReadyNAS box, a 300GB external drive, and a 500GB external drive. That gives me enough room to back up all of my files with a bit of room for versioning.

So here's how I envision growing:

1. 2.3TB (2TB accessible) RAID 5: Personal files and photos 2. 2.0TB ReadyNAS, RAID 0: Backup of personal files and photos (have to buy this) 3. 1.6TB (1.2TB accessible) ReadyNAS, RAID 5: Media, music, and video 4. 500GB + 300GB external drives: Selective backups of #3

Complicated. I need a live backup of #3, but I don't want to buy two boxes at once. Argh.