Moving Databases

I have two Postgresql databases - one on machine a, one on machine b. 
Both are currently running.  I want to put a copy of the database on a
onto machine b.  I know I have to stop the postmaster (obviously) on
database b, but what else is there to do? Which directories do I have to
move (if that's what I have to do).  Sorry - I know this is probably
simple, but I'm rushing to meet a deadline.

-- 
Jeffrey Napolitano
Lowly Intern
Software Emancipation Technology