Welcome to the Apple Support Communities
First of all, you have an Intel-based Mac, not a PowerPC-based Mac.
To back up your hard drive, you can use Time Machine and/or Carbon Copy Cloner to do it. Both have got pros and cons, that you can see in this site > http://pondini.org/TM/Clones.html
Personally, I recommend you to make two volumes on your external drive (or buy two external drives) and make both Time Machine and Carbon Copy Cloner, so you will get the advantages of both applications. For example, if your hard drive gets damaged, you can easily and quickly restore your backup (clone) made with Carbon Copy Cloner because it creates bootable backups of your hard drive, so you can restore your data with Carbon Copy Cloner or with Disk Utility's Restore feature.
With Time Machine, for example, if you erase the hard drive because you want to upgrade Mac OS X and you want to restore files from the backup, you can restore the files you want of your backup (applications, user files, documents, downloads, movies...)
While I agree with mende1 on having multiple backups I don't agree with recommending using one EHD for both Time Machine and a bootable clone. This is unwise because when (note not if!) the EHD dies you will have lost all of your backup. It is much wiser to purchase two EHDs and use one for TM and the other for your clone.
Also MacWorld writes excellent articles on backing up regularly, if you can get a copy of the March issue there are some excellent articles to be read there on backup strategies.