Tried to use Xingtone http://www.xingtone.com 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

Cheers