Tried to use Xingtone but no dice with Treo 600 format. Then I discovered it's .mid format, so any MIDI converter will let you create your own. You will still be unable to send yourself or synch a .mid file. So, setup your own site with just links to the mids (<64K) stored in the root of the site, navigate your Treo 600 browser to it and click the link!!!!
Create your own now the long way, later when updated use Xingtone