Help needed to set the Optimal Value for shared_buffers.

Hi,
  I have a couple of issues.
  One is regarding a file called 'fsync' and the other is related to
the sharedBuffers issue.

  1)  I am new to administering postgresql. When I was wandering
through the configuration files for the PostgreSQL, I observed the
'fsync' file. This file is almost identical to the postgresql.conf
file, but with differing values for parameters and some new / deleted
parameters when compared with the postgresql.conf file. What is the
file for?  Does it have any specific importance to be taken care of?

 2)  What is the optimall value for shared_buffers parameter to be set
for a huge database that contains a table with upto '4GB' data. We
have a dedicated server with 2GB RAM. The maximum connections that can
be made to the server is around 500. I found a lot of variation
between opinions of different people on this value. I also observed
that the said optimal values vary greatly with the version. For
PostgreSQL7.X, some suggested an optimal value of 32MB RAM and for
PostgreSQL8.0 some suggested a range of 32MB - 480MB. We are using
PostgreSQL7.x.

 Any help will be highly appreciated.

Regards,
Kishore.