Oh, Photoshop ! I too have 1.5GB of RAM, and have been able to easily cause swapping to occur. I am no PS pro, and just a mere amateur when using it.
The hard drive is a big bottleneck in a PowerBook. I know, as I have a 1.5Ghz PB 12". The PowerMac has 7200 rpm hard drives, which makes for a big difference.
Have you checked your paging activity? How big are the images you are working with?