Jdong and the others are right - even though you may get your desktop environment or window manager down to using less ram, as soon as you bring up a gui app of any significance, you are going to be thrashing the disk again.
This is why I cringe when I see the recommendation to just run Xubuntu for 64 or 128mb machines. It isn't a panacea, even though your desktop might be a bit faster.
In the end, to make up for minimal ram, there is nothing like the commandline, OR possibly apps that use ncurses like nano, mc, etc. Run the default TWM window manager if you like, but you'll still have to find the proper balance between your "desktop" and the ram that an app uses once launched and running. It all depends on your needs.
You could run these inside a minimal X environment from within xterm, or some other terminal application. Need just a bit more gui? How about using "leafpad" as your editor. Don't like xterm or their equivalents? Perhaps xfce4-terminal will do. Does Brasero eat up too much ram? Use wodim in a terminal.
Getting Ubuntu (or any flavor of Linux) running a low-mem ppc box might mean disappointment for quite a few unless they look at the box from a different viewpoint - the low memory will force you to become familiar with commandline apps and setup to make your ram usage the most efficient.
This alone is a GREAT purpose for low-mem machines if you want to learn the cli. There just aren't that many other useful gui distractions to keep you from hitting the books. :)
I don't want to start a cli-thread, since that has been covered elsewhere, but from a practical point of view, that is where one is headed with low-memory. I look at it as a positive thing really, but the cli isn't everyone's cup of tea.
There are a few threads about everyone's favorite cli-apps around - oops forgot another one for music: Aside from my favorite ogg123 inside vorbis-tools, many like MOC since it is an ncurses-gui. There are many others. Elinks might be useful for web-browsing, but if it is too sparse, it and even Lynx make for nice ncurses-based minimal local file managers, ie with *lynx /usr/share/doc* etc. Oh, forget Gimp with 64mb. Try installing imagemagick, and then looking at your jpgs with *display wedding.jpg* ...
I could go on, but there is a whole world of ncurses based apps one can run in a gui terminal if they are adverse to a raw commandline.
...just some thoughts....