I can't seem to set the -B option on the postmaster over 3000

I have Postgres 6.3.2 installed on a PII-400MHz with 256 Meg of RAM and
RedHat 5.1. I have recently learned about the -B option on the
postmaster command to increase the number of shared memory buffers.

I have managed to get the number up to about 3000 (i.e. 24 Meg) but
can't go over about 30 Meg. Since my system has 256 Meg of RAM and 512
Meg of swap space, why can't I go much higher with this?

Is there some upper limit for Postgres and shared memory buffers?

I thought that by maximizing this number I could get better search times
for my queries.

-Tony