Back when OSX was coming but not yet shipping, I decided to get a head start and run PPCLinux.
To answer a few of the questions so far:
Yes, you can dual-boot (or triple, with OS9) on a Mac.
Linux is derivative of UNIX; many commands are identical while others are different. So, even though you may recognise Linux commands in OSX, don't assume they're all the same.
I used Mandrake, LinuxPPC and YellowDog Linux. LinuxPPC is gone, but YellowDog is the best distro to attempt first anyway. It's RedHat based (so for the most part, if you know RedHat you know YellowDog).
OSX is quite a bit more functional than Linux for an ordinary desktop user like most of us; so I don't have a Linux distro anymore. However, if you're learning it at school or do IT work with LinuxX86 then it's worthwhile to go ahead and plan an install.
You need to totally reformat the HD so a full Mac backup is required. New users will find documentation the biggest hurdle; it's spotty and assumes prior knowledge.
I actually have a YellowDog T-shirt they sent me as a thank-you for writing some documentation for them. (Free software companies won't pay much for that kind of help; a T-shirt is pretty impressive as renumeration goes with Linux companies).
You can download the distro for free; burn it to a bootable CD, and go about it. I suggest heavy reading and having a printer for documentation, because you will need it when you're installing and configuring. I have a binder somewhere with about 300 pages printed out.
It's best to avoid recent hardware if you want to get it running; a G5 probably won't have things like video drivers available for a while. The Apple-OEM ATI or nVidea G4s should be OK.
There are vesions of Linux that run on most any Mac hardware; a very few machines are not supported. The basic system that is supported is:
Any PowerPC Mac with PCI card slots.
Older 68000 series Macs (like a SE-30) can also run certain distributions.
Although few people know it, Apple used to sell a UNIX-OS for Macs; it died with the PPC platform.
Yellow Dog Linux
Penguin PPC
If you are serious about installing, I strongly suggest getting on a PPCLinux mail list; newby questions may not be tolerated much but there are some very helpful people on them, including developers themselves.
It's probably worth your while to buy the distro and documentation from YellowDog for your first install, but it's available for free if you're adventurous and have broadband (or a small fee, without printed documentation and install support, from a bunch of suppliers. It's legal to burn and sell Linux CDs or CD-Rs under most distro licenses).
If you find installing and configuring Windows daunting, you won't like Linux. However, once you are up and running with it, you may prefer it to any other OS.
It's actually easy to configure if you have enough experience with it; the learning curve is steep though.
If your educational or professional interest may be more with UNIX than Linux, or covers both, you might also consider partitioning a drive for Darwin (BSD UNIX-based, available for free from Apple for PPC and X86). Pretty much all the same SW is available or can be compiled for both.
[ November 24, 2003, 01:28 PM: Message edited by: gordguide ]