If you have to reformat a hardrive and want to get your fully set up OS 9 system back after the reformat, here's how. First back up the folder named "System Folder" that contains the OS 9 installation you'd like to save. After you have finished installing OS X, just drop the System Folder back onto the hard drive and restart to recover your old installation. OS 9 should work flawlessly after the restart.
[Editor's note: I haven't tested this hint myself, but it makes sense given that OS 9 installations could be copied via drag and drop. I've also used this method to put my "optimized" Classic folder on various OS X hard drives; just drag and drop, and then reboot on the other drive and fire up Classic. Of course, if you're going to reformat a drive, you should be running a full backup of your drive using one of the available backup programs!]
Mac OS X Hints