Cannot allocate greater than 28 Meg of shared memory buffers

I have the new Postgres 6.4 installed. My system has 256 Meg of RAM with
512 Meg of swap space (PII/400 MHz; RH Linux 5.1). I am trying to
maximize the number of shared memory buffers for the postmaster. It
seems that if I get over 28 megs (-B3500), that the postmaster won't
run:

nohup postmaster -i -B4000 > regress.log &
[1] 737
[postgres(at)bigred ~]$ IpcMemoryCreate: shmget failed (Invalid argument)
key=5432001, size=33719840, permission=600
FATAL 1: ShmemCreate: cannot create region

I think that with 256 Meg of RAM that I should be able to extend that
shared buffer size even further. Is this a limitation of Postgres or is
this just that my system only has 28 meg to spare? (Or am I doing
something wrong).

Thanks.
-Tony