How to increase shared mem for PostgreSQL on FreeBSD

My system is FreeBSD 4.3 with 1GHz P3, 1GB RAM, SCSI RAID 5. I read an
article on PHPBuilder about optimizing PostgreSQL - one big thing was
increasing shared memory. I asked on the FreeBSD mailing list and got
different suggestions as to how to increase the shared memory - my main
question is - do I need to rebuild my kernel? I used sysctl -w
shmall=131072 to up my shared mem, then edited postgresql.conf to show:

shared_buffers = 15200
sort_mem = 32168

and postgres wouldn't start. I assume that means the shared buffers is
actually larger than the amount of shared mem, so it appears the sysctl
didn't really do anything. If I comment out the shared_buffers line,
postgres will start, but upping the sort mem doesn't help my insert
speed. Can anyone help me out with how to get the shared mem increased
so I can up the shared_buffers in postgres? Thanks,

Joe