Martin, I forwarded my question to my crystal ball, and it said you're like pampering endangered species.
That said, bringing up the question is almost answering it. There is a minor risk in updating in general (after all, it is software isn't it) and the risk is increased for a not supported application. You either take that risk and keep on updating, or freeze your configuration. Note that some other application might need the update.
I am in the IT business and have learned never to update/upgrade without previous testing (people were fired for less). You could go into that scenario as well if you like. So how will you solve this?
Buy a firewire disk, install OS X and CO 3.7.9 onto it, boot your PowerBook with it when an update is released. Install each update on this system first and test its performance. If alright, install the update on your internal disk.
You could reverse this scenario in that you install your working setup onto the external drive and store the drive until something went wrong with your internal drive and you have a backup to restore. There are several alternatives to these scenarios to come up with.
The disadvantage is that you may forget to live, love and make pictures (put your own priority here). You might start looking to CO 4 in the near future after all.