One Database per Data File?

I'm a complete newbie to postgres so please look the
other way if these questions are really stupid.

Is it legitimate to have one database per data file? For
organizational and backup purposes, I'd like to keep the
database files for each of several projects separate.
This means, e.g., that postmaster must have multiple
instances going simultaneously?

I'm thinking the answer is NO because, for one, the TCPIP
connection seems to be to ONE instance of postmaster which
then sorts out which database objects are in its container.

Am I close?
--
% Randy Yates % "Maybe one day I'll feel her cold embrace,
%% Fuquay-Varina, NC % and kiss her interface,
%%% 919-577-9882 % til then, I'll leave her alone."
%%%% <yates(at)ieee(dot)org> % 'Yours Truly, 2095', *Time*, ELO
http://home.earthlink.net/~yatescr