i was considering a DROBO as well, until I realized its actually a big external scalable storage device.
For backup I went with two external firewire drives, and Superduper software. One is connected to the iMac and the other offsite. Every two weeks or so i just switch the drives. If and when I get close to filling them I'll buy bigger drives.
Oh the dlink in my signature , is for my windows workstations...