[Zope] looking for advice to fine-tune large AT migration

Hi, I've got about 120K objects in my plone site, and I need to migrate from AT 1.3 alpha to the latest AT. I tried it last night, and the first step, migrating schemas, took all night. There is still the catalogs and references to migrate. I was quite conservative with transactions. I was doing a full transaction every 250 objects. Any suggestions on numbers to use for subtransactions and full ones? My box has 3GB RAM and a fast P4 CPU. Thanks, Gerry