Welcome to LQ. While Fedora Core 3 might be considered overkill for a simple file server, I'm sure it will have some nice GUI wizards to walk you through the process of setting it up. Regardless of which distrobution you use, you will want to familiarize yourself with Samba. You didn't mention what OS your wife's PC was running, but I will assume it is Windows. Samba allows file and printer interaction between Linux and Windows PC's. As for your Power Mac, if it can speak the Windows network language, then I'd assume it could also interact with Samba as well.
One word of advice if you decide to go with Fedora Core 3. After installation (or during installation), make sure to disable any unnecessary deamons that you won't be needing (like Apache, for example). Some distros install with a lot of extra stuff running that you won't need.