Upgrade Hard drive

see this similar question - you can refer to the 20" instructions they mention

the HDD is pretty easy to install - after you work your way threw the tear down the iMacs (majority of them) use just a standard SATA 3.5" HDD. (also the 20" and 24" depending on your model are pretty much the same with just more screws really - as for the tear down well....for my first one i used the tear down from here - ifixit.com - so just make sure you have a print out if this is your first tear down the reference is good to have so that you don't break anything or miss any screws). Back to the HDDs I think you can go up to the 1TB hdd without having any issues with the system's bios. I have upgraded 3 iMacs' hdds and all 3 they had no problem (oh side note - make sure you reconnect the heat sensor on the hdd, otherwise you can get false overheating alerts - i don't know if older iMacs have them but mine did).

Once you install the new hdds and reconstruct the iMac with all of it's million screws, you can either simply run the OS Restore Disks or if you want to just image the hdd you can use the program Super Duper -SuperDuper Link

SuperDuper is a pretty good program i used it on my MacBook Pro 13" to upgrade the stock 160GB hdd to a 500GB 7200RPM drive, and i used it on my iMac 24". My only down side is that it's somewhat slow even using a firewire 800 hdd enclosure - but then again the fact that the software is free it makes the wait worth it.

**NOTE** Make that if you are going to use SuperDuper to image the drive you do the image before installing the new HDD. The software program requires a working computer to work properly.**NOTE**