Modifying postgresql.org file

It has been suggested to me to increase my work_mem to make queries
preform faster. I believe I do this in the 'postgresql.org' file. I
seem to have two of these files:

/etc/postgresql/7.4/main/postgresql.org
/usr/share/postgresql/7.4/postgresql.conf.sample

I believe the second one is an example/sample file (similar to how
xorg.conf comes with a sample file). Can someone point me in a direction
as to where I can learn how to modify the postgresql.org file to
increase work_mem? I have the O'Reilly book "Practical PostgreSQL" but
it doesn't seem to cover this topic. I'm fairly confident that I can
tweak the postgresql.org file; given that I was successfuly tweaking my
xorg.conf file.

Any help/guidance is very much appreciated.

-Ken