I looked at those topics earlier. It seems that people are able to boot off of a USB external HD. My I can switch to a USB or eSATA connector if I wanted.
Right now I think my best choice would be to make my External HD for backing up ALL of my MacOS.
Then, I think I'll partition 80 GB of free space in the Internal HD, and set it as "Free Space." So when I get to the partition chooser of the Ubuntu Install, I select Largest Continuous Free Space. Largest Continuous Free Space applies to a partition that has NOTHING on it except space, correct? So I don't run the risk of having it install on my MacOS partition, because there is over 80 GB free there, even AFTER I partiton it?
So if I select Largest Continuous Free Space and let it do an automatic install, Ubuntu will set up a swap for me.
Then when it's done and I reboot, I reboot into rEFIt and select the Legacy (or Linux) icon? This is the part that got me confused. Do I select the Linux icon or the BootCamp Windows looking icon labled Legacy OS? I don't have any Windows softeware on my computer.