shared_buffers performance

Hi all,
I started to do some performance tests (using pgbench) in order to
estimate the DRBD impact on our servers, my plan was to perform some
benchmarks without DRBD in order to compare the same benchmark with
DRBD.
I didn't perform yet the benchmark with DRBD and I'm already facing
something I can not explain (I performed at the moment only reads test).

I'm using postgres 8.2.3 on Red Hat compiled with GCC 3.4.6.

I'm using pgbench with scaling factor with a range [1:500], my server
has 4 cores so I'm trying with 16 client and 4000 transaction per
client: pgbench -t 4000 -c 16 -S db_perf. I did 3 session using 3 different
values of shared_buffers: 64MB, 256MB, 512MB and my server has 2GB.

The following graph reports the results:

http://img84.imageshack.us/my.php?image=totalid7.png

as you can see using 64MB as value for shared_buffers I'm obtaining better
results. Is this something expected or I'm looking in the wrong direction?
I'm going to perform same tests without using the -S option in pgbench but
being a time expensive operation I would like to ear your opinion first.

Regards
Gaetano Mendola