I am a fan of Yellow Dog on my ppc machine and Ubuntu on my Intel boxes.
The benefit of Yellow Dog for me was always that they were the only major PPC distribution, which meant they are great at handling Macs. For your Intel machine, I would recommend trying some of the many "Live CD" distributions. You simply boot on of them from the CD and try it out. It will run everything needed from that CD without needed to be installed on your machine. Once you find the one you like, you can install.
Most Linux distros are under 5gb, so you will not lose a whole lot of space having one on the machine. When you install Linux, it will set up a boot loader. Each time you turn on the Macbook, you will get a choice of Macos or Linux. You will want to spend some time learning about the initial partition setup, but most distros will use your free space to set up the linux areas.
Here is one caveat that drives me crazy....no linux drivers exist for the Broadcom cards used as Airport Extreme cards. I have messed around trying to get my Powerbook to run wirelessly over and over again with no success. This is a major issue for me, and has meant I cannot move my Aluminum Powerbook to full time Linux use. I keep a TI Powerbook around which serves well.