It's the RAM more than the MHZ. Running this on my 867 MHZ powerbook with 768 MB of ram and it runs great even in crowded areas. My GF has less RAM on her powerbook, like 512, same processor, also works. Just MAKE SURE you have enough RAM and the 32 MB VRam requirement, if you don't meet both of those you might have problems, but if you have a G4 and the memory it will be fine. I know from experience.
Oh it MAY get really choppy, you might be unable to move for minutes or even hours at a time, and the game may not let you move smoothly instead causing you to jump around a bit, don't be alarmed, its not your macs fault, its the servers from hell reacting to overpopulation. If you get in when noone else is you'll see the computer is running it just fine even in crowded areas.