As first "addition" to your new iMac, seriously consider getting an immediate external HDD device. This hardware can be configured with "included" Mac OS Time Machine application. External HDDs can be connected via firewire800 (super fast) or USB 2.0 (average fast). If into backups, I'd focus on Fireware800 box.
To me, backups should come after OS and base Apps. After the backup device is "in production", one then starts creating their data. Backups should never be the typical "
oh - I'll get backups later..." things in life...
I hear some folks use "Western Digital 1TB My Book Studio Hard Drive" (re:
http://store.apple.com/us/product/TX879ZM/A) and are very pleased with it. Regardless of one's external HDD, do shop around. Selling prices do dramatically vary. Especially when compared to Apple Store items.
Hope this helps...
.