FreeBSD 7 needing to allocate lots of shared memory

I'm trying to get postgressql running on FreeBSD with 64 GB of memory.
Postgres will the the only thing running, so I want it to use as much
memory as needed.

So I increased shared_memory to 50GB, and also set:

kern.ipc.semmni=512
kern.ipc.semmns=1024
kern.ipc.semmnu=512

in loader.conf and:

kern.ipc.shm_use_phys=1
kern.ipc.semmap=512
kern.ipc.shmall=65536
kern.ipc.shmmax=68719476736

in sysctl.conf, and gave it a reboot, and checked the settings after reboot.

But  now, when I try to start postgresql, I get this in my log:

FATAL:  could not create shared memory segment: Cannot allocate memory
DETAIL:  Failed system call was shmget(key=5432001, size=54866198528,
03600).
HINT:  This error usually means that PostgreSQL's request for a shared
memory segment exceeded available memory or swap space. To reduce the
request size (currently 54866198528 bytes), reduce PostgreSQL's
shared_buffers parameter (currently 6553600) and/or its max_connections
parameter (currently 259).
        The PostgreSQL documentation contains more information about
shared memory configuration.

According to the setting I have made, I would guess that it should be
able to allocate this much memory. In fact, it only starts if I set the
shared_buffers to as little as 32MB!! So there is clearly something
wrong here... I hope you can tell me what.

PostgresSQL version 8.3.5, compiled on a fresh install of FreeBSD 7 with
no special options.

Kind regards,

Christiaan Willemsen