I had 4x250GB drives in my old machine but it was too small (half was a mirror).
Now I have 1x 500GB drive in the main desktop machine
1x 500GB Firewire drive backing that up.
1x 250GB USB drive plugged into the wireless N router (nas shared archive storage)
1x 250GB USB drive backing that up
1x 120GB drive in the laptop
1x 250GB firewire drive backing that up
It's a bit wasteful though so I plan on switching to a Drobo:
www.drobo.comAnd all my data will be on it via the router, as will my system backups.