I would try Yellowdog or Ubuntu for starters...
Yellowdog is a PPC only flavor of linux (so it supports mainly Macs and IBM servers) that is based on RedHat, they just released v.4.1 which has tons of support for powerbooks (my backlit keys even work standard!) Also, it has great sounds support which is nice. And it has a nice easy GUI install (this is the linux that I am using)
The current stable build of Ubuntu 5.10 is not ready for PBs at the moment but 6.0X (Dapper Drake (unstable)) is pretty good. Ubuntu is an awesome OS but because they are mainly x86 there is not AS MUCH pressure to fix all the little things that don't work on apple harware (for example).
I should let you know that with all linux flavors you will not have Airport out of the box because the makers of the airport drivers will not release them to the open source community . They are trying to reverse engineer them with some success but you will have to get pretty familiar with linux to get it up and running. Hope this helps... if you have any questions post them here and I am sure that you will get some help!