would provide more processing power than your current PPC machine.
However, why not consider one of the iMac machines, one with a 24" LCD screen, which would be head and shoulders above what you are currently using.
Anything you purchase will have way more processing power and memory than the existing.
Any new machine that you get will not have SCSI ports so if your current HI-Res printer is SCSI, it will need replacing. Unless, of course, you keep the old machine on your network as a print server.
Adobe has not yet released their Universal Binary version of Photoshop. Your new machine would (should) run your old version of PS quite well, faster than you are currently running it, but it will not be as fast as the new UB version which, they say, is due out shortly.
I'm using an old Mac Power PC to print high-res art prints. Not much memory or power, but its a real workhorse. It is getting pretty old, uses SCSI and I'm thinking of replacing it soon. I can do some Photoshop on it, color correcting mostly - I do the heavy work on a PC.
So, I would like to upgrade. I don't think I need anything too powerful, as I can continue working the way I do now... although I do I need reliability and some degree of processing power.
I don't have a lot of money, and would need to spend some of it on a decent display (I'm using an old 19" CRT, which is getting a bit wonky lately). I'm wondering if a Mac Book or Mac Mini would work for me.