I've been using something like the current setup for close to 2.5 years, so things are a bit foggy in my memory. The system was faster when I was working on billboards. For general Photoshop use though? I can't really remember. That's why benchmarks are useful - they make me feel better about my decisions. If you're only using Photoshop to add "I CAN HAZ CHEEZEBURGER" to a picture of your cat or toddler, there's probably not much point.
There's no real reason to keep your OS and Apps separate. If you haven't, read the Mac Performance Guide, specifically this pages about general setup and Photoshop setup.
Also, that setup doesn't have any backup drives. If you have 10.5, I'd use TimeMachine. It does slow my G5 down for a few minutes when it runs, but it's only a problem when I'm playing games. I just turn it off for that.
Once again, thanks for the info.
Mike