Auto Backup to External HHD - Older iMac

[Arun]

I am looking for a good hardware and software solution to backup my Apple notebook that is 3 years old (my daughter uses it). It is running OS X 10.4.
I understand that the new operating system has built-in backup utility. However, I would like someone to advice me on an economical solution that I can use to make sure that the internal harddrive is fully backed up seamlessly to this external device. I have purchased Seagatre's FreeAgent hard drive. But it does not come with any backup solution for Apple notebooks.

Thanks!