I have a friend who owns 2 Apple retail stores and he stopped selling the Drobo because of terrible tech support and lots of problems with the device (DOA and corrupted files). Pass on the Drobo. A RAID 1 device that provided redundant back up may be a cheaper and faster alternative.
Use SuperDuper or equivalent to weekly clone your iMac HD to a similarly sized external HD. This way you will have a ready bootable backup. Opt for FW800 over USB for the best transfer speeds. For data files and more frequent backups, based on your current HD size, using an external drive as a dedicated Time Machine backup is a great way to "set and forget" your data files backups.
I suggest that you check out Other World Computing (OWC) for highly rated, excellent quality external HD that support triple interfaces (USB, FW400 and FW800). You can buy the single drive units with up to 1TB of storage. If you are handy, you can buy the enclosure and then shop the web for the best deals on internal HD's and then assemble the unit yourself and you can save about $75. I built 4 such enclosures using only Seagate Barracuda drives of the latest generation. I've found great prices on these from www.newegg.com
. Knock on wood, I never had a failure with these units. I previously used Lacie drives and I experienced a number of failures on the electronics and powers supplies, but not the HDs.
One final note. Always, always, always have an external backup that you rotate to a different location just in case you have a fire or flood. I use a safe deposit box at my bank which is about a 10 minute drive from my house. I rotate 3 1TB drives through to have offsite backups of all of my images, videos and personal documents, statements, etc. (scanned images).
I hope you never have to rely on this, but if you do, you will be glad that a lifetime of images are not lost forever. You can always buy new "stuff", but the photos (and videos) are priceless.