Problems tuning shared_buffers value

I'm new to this list.

This morning I was experimenting with some tunings for PostgreSQL.
(I currently have 7.3.1 installed ... it's behind a firewall so I've
been using that as an excuse not to worry about 7.3.2 yet)  I was
curious about some things, so I set shared_buffers to 16 (which the
config file claims is the lowest allowable value)  The result was
that Postgre refused to start.  I upped it to 32 and the result
was the same.
I looked in both /var/log/messages and in /usr/local/pgsql/logfile
and found nothing about _why_ it wouldn't start, so I became bored
and commented out shared_buffers (returning it to its default value,
I assume) and the postmaster started just fine.
I know it's a little weird to be setting shared_buffers that low,
but this is an old (almost anchient) machine used for testing and
I was wondering how memory-efficient I could make Postgre, even at
the expense of speed.
I guess the real question is: what is the true minimal value for
shared_buffers.  Also, shouldn't there be somewhere I can get a
more descriptive error message than "postmaster did not start"
(in case I had changed multiple values and wasn't sure which one
was wrong)

TIA for answers.

--
Bill Moran
Potential Technologies
http://www.potentialtech.com