This is great! In fact, i got an Imac G3, slot loading with 128MB RAM (Yikes) and as a matter of fact I did this a few days ago.
What I did was to update my firmware to the latest version, burned a fresh Debian PPC CD (which is very very similar to the Xubuntu Alternate), and then ddisabled journalling.
Then I rebooted my machine. then I pressed C to boot from the Debian CD, then I went through all the processes until it came up with the Partitioner-
I hit "go back", and then "excecute a shell", then I typed
and then I resized my partiton
(this should be clear which partition you want to resize
resize MINOR_NUMBER_HERE START_BLOCK_SIZE_FROM_PRINT_OUTPUT NEW_END_BLOCK_SIZE
which would look something like this
Where 3 is the number that corresponds to your HFS+ partition, the second number is the position of your Partition [usually in MB- divide this number by 1024 to get it in MB]
resize 3 128.032 37237.821
And the end size is where you want your partion to now end.
Don't forget to write the changes to disc, or your partitions won't change.
If you want, install Debian in the free space, but I clicked Abort the installation, popped in the Xubuntu 8.04 Alternate CD, and then typed "install", which took me to the Xubuntu Installer. I folowed all the steps up to the partitioner, where there should be an option to install in the free space. i selected that and was as good as home.
EDIT: This is the iMac model that is tray loading, I presume, so it may have a limit. Be sure to have ALL your partitions within 8GBs.
EDIT 2: i don't think Mac OS 9 Had journalling.