Admin questions

I installed Postgres 7.1 client and server on my Redhat 7.2 box

I only allow SSH connections and I want to set up accounts for the users

of my box.
The only client that will connect to this database will be the one
installed on my box.
(They will SSH into the box, then log on to psql..that sounds, right?)

How do I set it up so that the PostgreSQL accounts have the same names
and passwords as the
home accounts on the OS? Is this possible?

Furthermore, if I have applications (JSP, Perl, etc) that query this
database, will there be conflicts?
Do I create a www user in this case for PostgreSQL stored procedure exec

privileges?

Thanks.