Glad you are enjoying your new iMac!
When funds permit, grab yourself an external FW800 enclosure and a 3.5" SATA-II drive (something like a WD Caviar Green 1Tb or 2Tb) and set up Time Machine to back up to it. If you want an enclosure which is ready to go, LaCie or OCW do them ready to go as well as blank enclosures if you want to choose the HDD yourself.