An easier question to answer.

Do you need to do ANY calculations that involve more than
one customer?

If a lot are needed, then go for one database.

If you only need to bill them <g> then create a database
with the billing information, and separate databases for
each client. Then when you select a new client, you just
change your login for the private stuff. I use IBObjects and
just load a new database path in my case, for each station,
and each of them are on other machines as well!

Backing up all those files can be a problem, but all you
have to do is set up a gbak command line command and
populate it from a scan of the billing database. ( We only
restore now ocasionally, but this could be automated as well
)

--
Lester Caine
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L.S.Caine Electronic Services