Actually, OS 9 is far
better for security than outdated OS X, simply because it isn't anything resembling a modern OS, and what there is to attack is so different from the normal exploit targets. And, it helps that there's no external attack surface either.
I mean, I've heard of, in
modern times, a NeXTSTEP VM's sendmail
getting used as the open relay that it was and spewing spam. Why? Because it's a *nix, and everything's in the place you'd expect it to be on a modern *nix, even if it's ancient, and because it was all servery, it did servery things.
Now consider that OS X is just a newer version of the
exact same OS. It's got all those services hanging around, not being patched, actively being exploited.
Really, OS X prior to 2 versions back from current is just as dangerous as XP. That means if you're running even Lion on a network connected machine, you're running a huge risk. And, I don't care if you have to run it to use your G5 or late G4, don't network connect it except when absolutely necessary. (You could always use Linux, too.)