Big Tables vs. many Tables vs. many Databases

Hi there,

I'm currently about to redesign a database which you could compare with a
database for managing a library. Now this solution will not only manage one
library, but 100 to 500 of them. Currently, eg. all the data about the
inventory (books) is held in one table for all the libraries.

Is it useful to spread this to one table for each library, by eg. giving it
an id as a postfix?

For one library, we currently need about 150 tables, so that would easily
increase a lot if there would be a set of this tables for each library. On
the other hand, there are only a very few tables (2-5), which are used by
all libraries. All the rest does not interact with each other - and don't
think about exchanging books betweens libs, as the library is only an
example...

One other solution would be to make one database for each library. What do
you think of that? Does PostgreSQL has any problems with that much tables?
Would it better to spread the data by databases?

Thanks for your opinions,
  Dirk