If you have an Intel Macbook, you are much better off running Windows nav programs on it. Roadnav is a free and open source mapping program, but it is no longer under development. It tends to be very slow, and although it works, it is not as good as a commercial product.
If you really don't want to spend any money, take a look at Navit. I think they have some kind of Mac install and it is under development now.
As far as GPS with Google Earth, you can already do this on the Mac. Search on terms Google Earth in the Mac forum. It also works but you will need both an internet connection if you are not caching the data, and it does not give turn by turn directions or auto reroute and so forth. You can set it up with Spaces and flip back and forth to it via a hotkey or hotkey combo with ICE3.
Thanks for your help and quick reply! :)