As suggested, make sure you do a complete backup before deleting existing archives. Double check the new backups to make sure you’ve got every thing you wanted to backup.
Note the difference between the above Time Machine suggestions and using the Apple Backup program. Time Machine will backup everything by default, and its archives will keep expanding to fill your external drive over time. You can tell Time Machine not to backup certain folders in Preferences. I exclude my System and Applications folders, since I can reinstall those from disks. Time Machine is great to go back and get previous versions of files that you may have discarded or altered.
Backup will perform manual or scheduled backups of specified folders to a variety of media, including your internal drive, CD/DVD, or to the MobileMe cloud. Good move changing from backing up to your internal drive. If (I should say when) that fails, you’d be screwed.
I use a combination of Time Machine for frequent, file-level archives and Retrospect for complete daily backups.
Note that Time Machine will give you bootable backups that you can use to restore your internal, if you don’t exclude System files.