why can't I increase shared buffers to higher value?

I have the following config but increasing the shared buffers to a value greater then 32 doesn't let the database server start (I want a value of 256MB there because I will have a giant table of 12 million rows which will be qeuried extremely).

I have a 3GB RAM amd64 system running freebsd 6.1 with:
 
Maintenance_work_mem is 32MB
Max_stack_depth is 3MB
Shared_buffers is 32MB
Temp_buffers is 8MB
Work_mem is 32MB
 Max_fsm_pages is 204800
 Max_connections is 3

And I am doing this:

sysctl -w kern.ipc.shmmax=1954311424
sysctl -w kern.ipc.shmall=16384

Whay is my shared buffer value not increasing?
Hope you can help me out or give me a few tips.
O yeah, I'm using version 8.2.3.

Thanks