install Linux on iMac G5
Debian is probably your best choice. The problem is that the Flash Player situation would be worse on Linux than on Mac OS X. There is no official Flash Player for Linux PPC. There are some alternatives, though, described in this article. This page has an extremely detailed guide on installing Debian for PowerPC. Also, this page describes which macs are compatible with Debian PPC. Oh, and yes, MintPPC would work.