I can say that I was running Photoshop on a 1.25 GHz PowerBook before I got my 2.0 GHz MBP.
The PB had 1GB memory and my MBP has 1.5GB memory.
Photoshop is noticeably slower on my MBP than it was on the PB. I have an action that I run in Photoshop quite often than resizes images. On the PB it would run through them: click. click. click. click. click.
On the MBP, that same action takes about 3 times as long: click.......click.......click.......click.....