Configuration settings (shared_buffers, etc) in Linux: puzzled

On a linux box (Linux db1 2.6.18.8-md #1 SMP Wed May 23 17:21:37 EDT
2007 i686 GNU/Linux)
I edited postgresql.conf and changed:

shared_buffers = 5000           work_mem = 16384
max_stack_depth = 4096

and then restarted postgres.  The puzzling part is that postgres
actually started.  When I have done this on other boxes, I had to edit
kernel settings to allow for more shared memory.

The kernel settings are currently:

shmmax: 33554432
shmall: 2097152

I would have expected to need to increase these before postgres would run.

I have a nagging suspicion that something isn't right.  Like it's not
actually reading the conf file, or I might have a problem on reboot.

I'm not sure if I'm worrying about nothing, or if something weird is
going on.