The only thing I've done in CS3 that's given me problems is Neat Image plugin sometimes goes to a spinning beachball when I change the camera profile after I've already applied the default profile. I'm a heavy user, and do a lot of compositing. I'd say do some system maintenance. Maybe check your scratch disk to see if there's some I/O issues or something? I'm on a G5 2.0 with 4GB RAM.
You might want to check and see how much free ram you have left.
Sometimes the pages-out aren't reset for a long time and they build up eating the free ram, which also slows down everything.
Restarting a computer every once in a while is a good thing.