Nice! I'm assuming you are installing the "server" version, which leaves you at just the commandline. This is actually good, since you'll be running the server-tuned kernel, but you want to put some X on it. The "server" images make setting up a LAMP environment easy with a simple checkbox during install.
You may have picked them up from here:
http://cdimage.ubuntu.com/ports/releases/
Some might suggest falling back to 8.04 for the possibility of longer updates with the LTS version (although with ppc, there are no guarantees anyway)
Although these docs target low-memory users on Intel, they are GREAT docs for doing exactly what you want to do with your server for ppc:
https://help.ubuntu.com/community/Installation/LowMemorySystems
In your case, you are already running the server so you are half-way home. All you need to do now is add the X components, and a window manager. Everyone has their favorite, and mine lately is IceWM. For server use, I see no need for a login manager such as xdm or gdm, but you can add those if you want to. They are all just an apt-get away.
However, you may still be facing some common /etc/X11/xorg.conf issues, which you'll have to manually edit.
For the B&W, you'll need to know what your external monitors vertical and horizontal frequencies are, and it's native resolution if using an lcd. Here is a recent response to another b&w user that might help:
http://ubuntuforums.org/showthread.php?t=1010570
and be sure to see the wikis since you'll probably have to edit in your video driver as well:
http://ubuntuforums.org/showthread.php?t=427714
Hope this helps. If things get too frustrating, I can also recommend Debian 4.0r5 as a server with a gui if it comes to that. Grab the XFCE4 Cd-1 for powerpc and during install you'll also see the options for a server. Doing what you want to do is very easy with Debian, and any reconfiguring of the X server for your monitor is also easy since it uses the older classical X server configurations.
http://www.debian.org/distrib/
Wow, sorry if this was info-overload. :)