FileMaker can indeed be used for all of the jobs that you mention. Looking at the samples at http://www.museumsoftware.com/screens.shtml PastPerfect looks like it is comprehensive, but still just a database, that could be duplicated by FileMaker. The catch is of course that it will take time to set up, but maybe you don't need something quite so comprehensive.
One of the good points of FM is that you can start with some basic tables and fields, and add to your database as needed. Try to write down everything that you can think of that will be needed, so that you have a plan of where you're heading. Then pick a place to start and jump in. When you get stuck, just come back here!
On your windows/mac point, you should not have any trouble swapping from one to the other, but watch out for different versions, as file formats changed at fm3, fm5, and fm7. Files convert up OK, but you can't go backwards!