The Mac is not so simple because installed applications store frameworks in the system library, etc. Still, it it easy to make a reliable backup, assuming that you have an external FireWire (or I suppose an USB) disk drive:
- Make sure that there is room on your external disk for your home directory and another 5 gigs
- Plug in your external backup disk, and restart with your install DVD
- Install OS X, choosing the external drive
- When the installer asks if you want to import previous user data, say yes and select your internal disk that contains your system and user account
- Reboot using your external disk and run software update
- Pack away your external disk as a backup of your working environment - preferably off site for extra protection.
Nothing is for sure in life, but external disks are likely to have longer shelf life for your digital assets than CDR-R and DVD-R media, but using both, and off site backup on remote servers, etc. is also a good idea.
Because of the (hopefully) very high reliability of external FireWire (or USB) disk drives, I also copy important Windows and Linux backup files to an external disk using either my LAN or DVD-R backups. Replicating really important files to multiple computers on your network using rsync is also a good idea.