Debian GNU/Linux PowerPC PowerMac Page
Here are some highlights of the PowerMac installation of Debian. For detailed instructions, please study the fine Installation Manual. The Debian Installer team have spent many long hours attempting to answer your questions in advance and give you excellent written guidance as you install Debian.
It is certainly possible, and there are actually quite elegant solutions for the NewWorld Macs, to dual boot your Debian PowerMac system with Mac OS and/or Mac OS X. If you are planning on doing a new installation of Mac OS X, though, do it before installing Debian. The Mac OS X installer is very unkind to existing systems when it installs. Shortly, you may also have the option of running Debian within a Darwin system.
The Debian installation uses a 2.6 series kernel. This kernel should support most PowerMac hardware. A 2.4 series powerpc flavour kernel is available for users with externally supplied modules that have not been ported to 2.6 kernels. However, the use of this kernel is strongly discouraged.
You will need to partition your disk; Linux must be installed on its own partition(s). If you have a single-disk system, that will entail backing up everything on your system and restoring it after partitioning is complete. Some third-party partitioning tools may be able to 'shrink' a partition so you have room for more partitions on your disk without destroying what's already there, but they will undoubtedly advise a backup also. Drive Setup does not offer that option, it erases the entire drive.
1 GB is probably enough space for an experimental Linux system. You can get by with less, perhaps as little as 400 MB for a really basic system, but you'll more than likely want more than just the basics.
After partitioning your disk, you will need to obtain an installation CD or download the installer system. Once you finally sit down to do the installation (preferably with installation manual in hand), you will probably burn up 2 or 3 hours making it happen. An experienced installer can get a basic install done in under half an hour.
If you're really uncomfortable with command lines, then install the X desktop after you've finished the basic system install. But give it a second thought, too: there's a world of power in the Linux command line. Some things that are very difficult to do in a graphical interface are very fast and efficient on the command line. Also, since the Linux system is command-line based, there are some functions which are only accessible from the command line. The default system sets up 6 command line consoles, and one graphical one. You can get a lot of work done in those other consoles while you're browsing the web... and that way you can have the best of both worlds.
The PowerPC platform runs really well with Linux. It's highly respected in much of the Linux world. Enjoy, and remember to give something back!
For help with Quik on OldWorld Macs, see http://penguinppc.org/bootloaders/quik/
For detailed information on the various Mac models and in particular info on working with OpenFirmware with any given model, check out the NetBSD PowerPC Model list. Since their installation requires one to get OpenFirmware up first, they're the experts on that angle.