Design Q.:logic in app or db?

I have general design question about Postgres usage:  How does one decide
how much, and what parts of logic should go in DB rules, triggers,
functions, constraints etc, versus what should go in the application? 

I see postings here from people who obviously have a lot of domain
logic in the DB side.  I currently have almost none.  I plan to set up
a bunch of RI constraints to keep things clean and consistant, but what
about logic that implements frequent domain operations?

Brief sketch of my project: 2 developers, 4k lines of python(gtk, pygres),
2 main GUI user apps and a few read-only scripts for web display, 
50 concurrent users(all local), DB performance important but not currently
a problem.

The main thing not done yet is to facilitate ad-hoc queries 
(via odbc excel etc.) from db-naive users:  maybe restructuring the
db to make it simpler, maybe views and functions... The data is
somewhat complex in structure.

-- George
-- I cannot think why the whole bed of the ocean isnot one solid mass of oysters, so prolific they seem. Ah,I am
wandering!Strange how the brain controls the brain!-- Sherlock Holmes in "The Dying Detective"