Tuning shared_buffers with ipcs ?

Hello,
I've seen a couple references to using ipcs to help properly size
shared_buffers.

I don't claim to be a SA guru, so could someone help explain how to
interpret the output of ipcs and how that relates to shared_buffers? How
does one determine the size of the segment arrays? I see the total size
using ipcs -m which is roughly shared_buffers * 8k.

I tried all of the dash commands in the ipcs man page, and the only one
that might give a clue is ipcs -t which shows the time the semaphores
were last used. If you look at the example I give below, it appears as
if I'm only using 4 of the 17 semaphores (PG was started on Oct 8).

Am I correct in assuming that if the arrays are all the same size then I
should only need about 1/4 of my currently allocated shared_buffers?

------ Shared Memory Operation/Change Times --------
shmid    owner      last-op                    last-changed
847183872 postgres    Fri Oct  8 11:03:31 2004   Fri Oct  8 11:03:31 2004
847216641 postgres    Fri Oct  8 11:03:31 2004   Fri Oct  8 11:03:31 2004
847249410 postgres    Fri Oct  8 11:03:31 2004   Fri Oct  8 11:03:31 2004
847282179 postgres    Fri Oct  8 11:03:31 2004   Fri Oct  8 11:03:31 2004
847314948 postgres    Fri Oct  8 11:03:31 2004   Fri Oct  8 11:03:31 2004
847347717 postgres    Fri Oct  8 11:03:31 2004   Fri Oct  8 11:03:31 2004
847380486 postgres    Fri Oct  8 11:03:31 2004   Fri Oct  8 11:03:31 2004
847413255 postgres    Fri Oct  8 11:03:31 2004   Fri Oct  8 11:03:31 2004
847446024 postgres    Fri Oct  8 11:03:31 2004   Fri Oct  8 11:03:31 2004
847478793 postgres    Fri Oct  8 11:03:31 2004   Fri Oct  8 11:03:31 2004
847511562 postgres    Fri Oct  8 11:03:31 2004   Fri Oct  8 11:03:31 2004
847544331 postgres    Fri Oct  8 11:03:31 2004   Fri Oct  8 11:03:31 2004
847577100 postgres    Fri Oct  8 11:03:31 2004   Fri Oct  8 11:03:31 2004
847609869 postgres    Fri Oct 15 11:34:28 2004   Fri Oct 15 11:34:29 2004
847642638 postgres    Fri Oct 15 11:33:35 2004   Fri Oct 15 11:33:35 2004
847675407 postgres    Fri Oct 15 11:34:28 2004   Fri Oct 15 11:34:29 2004
847708176 postgres    Fri Oct 15 11:27:17 2004   Fri Oct 15 11:32:20 2004

Also, isn't the shared memory supposed to show up in free? Its always
showing as 0:

# free
             total       used       free     shared    buffers     cached
Mem:       3896928    3868424      28504          0      59788    3605548
-/+ buffers/cache:     203088    3693840
Swap:      1052216         16    1052200

Thanks!