Macintosh Settings for Second Life
This is a good start but you can adjust your settings according to your own system.

You don't need more than 300-350 on the Bandwidth

Max out your Graphics Card Memory. Mine only goes up to 64MB, if yours goes higher choose that button.
You can also optimize graphics performance by downloading ATi Displays app if you have a Mac with an ATi graphics card like most of the PowerBook/iBook series. Set Second Life.app to High Performance. DO NOT UTILIZE THE OTHER FEATURES AS SOME LIKE THE ROTATE COMMAND WILL LOCK-UP YOUR DISPLAY FORCING YOU TO REINSTALL MAC OSX.