I installed the 3.0.1 "Sirius" version on my PowerMac 7300/200 (equipped with a 2 GB HD and 96 MB RAM, pretty much the original configuration). Had to install a small MacOS though, as these are "OldWorld" machines that cannot boot directly into Linux; you need to boot into MacOS then, using BootX, boot into Linux. This is an added step to the installation and booting process but once you're done it works like a charm (BootX boots Linux by default, so you can press the power-on key, walk away, and when you get back you're into Linux).

I chose WindowMaker as my window manager as it's very light on the machine (disk space and CPU usage-wise) and it's basically a copy of NeXTStep, so it's much more appealing to a Mac user than KDE or GNOME (which are horribly Windows-looking). Xfce may also be a solution if you're looking for a lightweight window manager.

As for the app side, everything I need is available, Yellow Dog Linux comes with a tool called YUM (the YellowDog Updater, Modified) which makes installing and upgrading applications a breeze. When a particular package I need is not available through yum (that is: not available on YDL's servers as a compiled app) I compile and install it by hand, which is not a hassle as it's usually done using the standard "./configure; make; make install" process.

Go to http://www.yellowdoglinux.com/ for more info. They are Mac-people working on a Linux distribution especially crafted for Apple hardware, so they really are the perfect solution for you.