There is a lot of "gray area" where running Disk First Aid, removing internet preference when IE5 crashes, extension conflicts that don't appear immediately, having Disk Warrior and TechTool Pro 3 handy. Faulty drivers, plain old "bugs." In Adobe or Apple products.
* Put your system on its own partition.
* Use scratch partition or drives (IBM 75GXP's are a good choice)
* Keep a "clean" copy of updated system in an .smi file - don't even rely on your "frozen" backup.
- Don't compress, allow writes and ample free space in the .smi for applying updates like MRJ 2.2.2 or other Apple updates
* Backup at least daily, if not more - takes less than 10 minutes to update backup drive using Retrospect or Synchronize! or something.
If the system does go, you can erase and replace with .smi and merge or copy - but always try to install.
Keep all the updates and downloads off net in one place if re-needed.
Keep systems to minimum of extensions, even Apple, and definitely 3rd party unless essential just for work - avoid "enhancing" if you want stability.
And keep a 2nd backup done monthly in addition to daily backup drive, and removeable or something for specific projects etc and stuff that can be off-line archived -- for next year's project.
Also consider looking into an excellent book for Troubleshoooting and upgrade tips and tricks - a couple have come out recently that are excellent.