I have a Powerbook G4 800 (with the 15" screen) and it's quite usable for Maya. However, because I started on the book before Maya for OS X was really ready to go, I have an Athlon 1.4 that I use for Maya when I can.

The Athlon is DEFINITELY faster. However, Maya on the Mac works pretty well, so you might just go with what you like.

One thing to note: using Maya on the Mac without buying a three-button mouse is virtually impossible. I have an external three-button mouse that I have carried around when I have been planning to do Maya stuff, although right now I do not use Maya on my Mac.

As far as a general purpose computer, though, the Powerbook is fantastic. I use mine all the time for just about everything except Maya. When I'm at my desk (like right now) I have the thing plugged into an external 18" LCD monitor, an external keyboard and mouse, and the power supply, and I can completely forget I'm on a laptop at all. Unplug a few connections and go to Starbucks and I'm surfing the web wirelessly. It's tremendously convenient. Sure you can do all of that with a PC laptop but the size and shape just aren't as nice, and I like the Mac OS.

-- Mark