[Zope] Large ZODB vs SQL + cataloging issues

Hi I am looking to build a system to hold 100s of sites, each of which holds several catalogues (as in on-line shopping, not ZCatalogs). How large can ZODB realistically go? At which point would it make more sense to hold data in a n RDBMS and use sql from Zope instead? Also, a related note is: If I have 100 online shops running from my Zope isntance, and one if them is a thousand times bigger than the others, how will the ZCataloging and general traffic on the big site affect the others? I'd like to just use the ZODB, but I have to convince some people that it will perform. Any case studies, tests, benchmarks, articles, etc would be very useful. TIA, Felix.