I'll make this easy:
Collector - Powerbook G4
Non-collector - Macbook
Why? Because the Macbook will run circles around the Powerbook G4. How do I know this? I have a friend with the exact same config that you're wanting to get. I compared my Macbook to her Powerbook G4, and it's not anywhere as snappy, or even opening up an app takes 2-4 times longer. Little things like that can make a difference.
The GUI also runs much smoother, especially in Leopard. The Macbook could stay at a brisk 60 fps scrolling through in Coverflow, while the Powerbook pauses sometimes, and the framerate is sluggish, depending on how many documents are in a folder, etc. So even though the Macbook has a crappy, integrated solution for graphics, the drivers are tuned well enough for the GUI of OS X. I'm guessing that slow 167 Mhz FSB of the Powerbook slows down the GPU quite a bit...
Anyway, point being, get something new. Getting something old is like getting an old computer that you already have, if you've got a computer like that...that is. 😛 I mean, get a Macbook now, then later, if you're inclined to, get a Powerbook. By the time you plan to get a Powerbook later, it'll be much cheaper later I imagine. Those PPC Macs are losing their value FAST.