PGSQL database size question

Hi,

I am currently brainstorming ideas for a new RIA that I am planing to
make.  In the RIA there would be many different sections which do
different things.  Some of these sections would require data from
other sections and some sections can sit on there own.  Knowing that
this RIA is meant to be easily to distribute and easy to resize,
should the RIA use multiple databases and few tables within each
database, or should there be only one database with lots of tables in
it?  I guess my question is, what is the point at which it is more
advantageous to use multiple databases?  One of the downfalls of using
multiple databases that I know of, is that it is much harder to
transfer/manipulate data between two tables in two different
databases.

Let me know what you think.

Thanks,
Daniel