shared_buffers Question

I've been running a postgres server on a Mac (10.3, 512MB RAM) with 200
clients connecting for about 2 months without a crash. However just
yesterday the database and all the clients hung. When I looked at the
Mac I'm using as the postgres server it had a window up that said that
there was no more disk space available to write memory too. I ended up
having to restart the whole machine. I would like to configure postgres
so that is does not rely so heavily on disk-based memory but, rather,
tries to stay within the scope of the 512MB of physical memory in the
Mac.

Am I correct in thinking that lowering that value of shared_buffers in
postgresql.conf will reduce the amount of disk space that is swapped
for memory?

I lowered the value from 2000 down to 500. Was that the right thing to
do or should I have gone the other way? Any other settings I should
look at? Thanks!