You could try out the Chromium OS. Its the open source version of the Chrome OS (read: unofficial), much like the open source Chromium Browser as compared to the official Chrome Browser. So it might be lacking certain features that the actual Chrome OS has.
The following video shows you how to install the Chromium OS onto a USB drive. Basically after installation, the machine boots up from the thumb drive to run Chromium OS (you have to change the boot option to USB while starting up), while not installing or overwriting anything on the internal hard disk itself (at least I think so). So it might be good for a trial run if you aren't sure that you want to overwrite your current OS.
http://www.youtube.com/watch?v=jY51qKLGFnE
Its also on wiki. The most popular version is by Hexxah, which is the one the above video attempts to install.
http://en.wikipedia.org/wiki/Chromium_OS
[Edit] Oops sorry didn't realise that your processor isn't intel based.
You could try powerpup or slackintosh for ppcs.
http://karkuta.drivehq.com/
http://slackintosh.workaround.ch/index.html