I can't speak for Macbook Pros but I went from a dual-core 3ghz PC notebook with 2gb RAM, 120gb internal drive and four external drives to a MacPro because the notebook couldn't keep up with Photoshop CS2 or CS3 Beta. My MacPro has 5gb RAM and four internal drives running CS3 and I'm pleased with performance.
I have Photoshop set to take 80% of RAM and it takes needs it to reduce (not eliminate) scratch drive activities. Photoshop CS3 is a very RAM intensive program. You can run it on a notebook but you want to max out the RAM and perhaps use external drives to provide scratch space.