I ran OS X 10.1 on a 604e/200 with 64MB of RAM. Talk about waiting on a miracle happening. It took about 4 hours to install, about 20 mins to boot... but once you got to the desktop it was steady but not all that bad, surprisingly. As long as you only ran 1 app at a time it was fine, and from rooting about in the terminal and enabling the Web Sharing and PHP, it was clear the underlying OS was running just fine it was just struggling with the desktop (which was all rendered in CPU at that stage, especially as it was running on a Rage 128).