I just went through this! Same computer, same idea...
If you haven't ran into it yet, most linux distros and a ton of programs that will not work because it has a PPC processor, instead of i386.
I've tried several things, but for the most part the only distro that is going to work nicely is Debian. Ubuntu is an inflated Debian that will not run well.
I suggest you get the "business card size" (it's like 30-40 MB)
http://cdimage.debian.org/debian-cd/4.0_r3/powerpc/iso-cd/
When you install it, you have one or two options... you can selected "Desktop" and "Standard" installation, that will get you most of what you need, and start you with GNOME.
Or you can do just the "Standard" system which will bring you to the console. If you have it hooked to the internet then you can use Apt to get a base system installed.
With the login type in "root" then whatever password you set for it at the installation.
Type
apt-get install xorg fluxbox dillo iceweasel leafpad
BUT
That's only if you want an absolutely bare system. Just doing a desktop would work.
I had trouble with GNOME, so using Synaptic I removed all of it, and put KDE on.
Works sweeeeet now.
By the way, "Firefox" isn't compile to PPC apparently, at least not in Debian repositories. The program is compiled, but it uses the name "Iceweasel" for some copyright nonsense.
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Linux Mint is an even more bloated version of Ubuntu, it won't run well at all.
OpenSUSE's RAM requirements are way higher than your computer could have, but aside from that none of the other distros (or this) will run on your computer because of the type of processor.