> Which approach is better in terms of performance: using one database
> file for all of the customers, or having 500 different databases,
> served by a single FB super-server process?

I would estimate that a single Database would be an good solution.

Within that database you can either create separate tables for each
customer or you can use an single table and add an customer-id to each row.

Regarding updates to metadata both strategies are ok: For updating
individual client tables you can generate an script.

If performance becomes an issue you should consider to use some kind of
application server which does concentrate the work on an lower number of
connections.

Elmar