The comments made here about Aluminium G4 PowerBooks is just wrong. Well, most of them. CS3 runs as good, if not better than CS2 on my PowerBook G4 1.33Ghz, with 1.5GB Ram. I also own the latest C2D iMac and MacBook and for everyday work there is not much difference. The G4 takes a couple more bounces before apps open, sure, but thats really not the best indicator of performance.
If you can find a relatively recent PowerBook at a good price, don't hesitate. Go for it. Ignore the recently converted Mac fanboys that have no idea of the real power of a G4.