Limited Shared Buffer Problem

Hi All,

I have a server running CentOS5 with 6gb of memory that will run postgres
8.3 exclusively.
I would like to allocate 4gb of the memory to shared buffers for postgres.
I have modified some kernel settings as follows:

shmall 1048576 pages 4,294,967,296 bytes
shmmax 4,294,967,295 bytes

I can set the postgres config to shared_buffers = 2700MB but no higher.
If I try shared_buffers = 2750MB the server fails to start with a message it
cannot allocate memory:

2010-01-29 11:24:39 EST FATAL: shmat(id=1638400) failed: Cannot allocate
memory

Is there some other setting that could be limiting the amount I can
allocate?

Excerpt from postgresql.conf:

# - Memory -

shared_buffers = 2750MB # min 128kB or max_connections*16kB
# (change requires restart)
temp_buffers = 32MB # min 800kB
max_prepared_transactions = 10 # can be 0 or more
# (change requires restart)
# Note: Increasing max_prepared_transactions costs ~600 bytes of shared
memory
# per transaction slot, plus lock space (see max_locks_per_transaction).
work_mem = 2MB # min 64kB
maintenance_work_mem = 32MB # min 1MB
#max_stack_depth = 2MB # min 100kB

Any help appreciated, Thanks

Rod