I have posted before that I couldn't get XPostFacto to run.
Now I was successful and want to share my experiences, because
installing OSX on a very old machine is absolutely not a one-click procedure. At least it wasn't in my case.
DO NOT MAKE THE MISTAKE to skip reading the install docs. If you are like me you usually skip all the bla bla and jump in to work, but that is not a good idea in this case.

I own a blue iMac from 2000 (350Mhz G3, updated to 128MB RAM).
Here's what lead to success:

Step 1)
XPostFacto on versiontracker i a dmg file, which is hard to open on OS9.
I posted a link as a response to my other comment for a link to XPostFacto.sit - this link is written in the documentation inside the dmg file, which I luckily opened on a different computer with OSX. If you have problems opening the dmg, use my link.

Step 2)
Before you start, read the documentation of XPostFacto carefully, because depending on your Mac you will need to have other programs installed beforehand.

Step 3)
In my case the iMac needed a firmware update. But before you do such an update, check your RAM with DIMMFirstAid. It will tell you if the updated firmware will acceppt your RAM or not. If you update and have RAM installed that does not live up to apple's expectations, you'll have a real problem.
Installing the new firmware is not as easy as installing a program. Read the instructions!

Step 4)
Insert your OSX install CD/DVD and start XPostFacto. It will restart and it will try to boot from the OSX installer volume. However, in my case, it didn't succeed. Instead OSX would display a stop-sign. If you, however, restart and press alt, you'll be allowed to choose your start volume. Now OSX will boot and install.

Step 5)
If you like, you can turn on the L2 cache. (See XPostFacto docs). It is not a necessity, though.


As a result, I am personally puzzled about the great performance OSX has even on a 350Mhz G3 with only 128MB of RAM. It is not slow, nor instable. But you should watch your free space and restart when runs out. Especially Mail should not be open while the HD space runs out of space (due to virtual memory swapfiles that get created easily with so few RAM).
If you (like my mom) do not use more than Word, Internet, Email, there is no need for a bigger config.