shared memory after server kill

I have a problem that I can't understand.

I am using macosx, pg 7.4.3. I had a server running and working. I
tried to bring the server down with 'pg_ctl stop', which failed (server
running on localhost). I then (timidly) killed the postmaster process.

When I tried to bring the server back up, I got an error similar to:

FATAL: could not create shared memory segment: Cannot allocate memory
DETAIL: Failed system call was shmget(key=1, size=1081344, 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 1081344 bytes), reduce PostgreSQL's
shared_buffers parameter (currently 50) and/or its max_connections
parameter (currently 10).
The PostgreSQL documentation contains more information about
shared memory configuration.

I have 4 Gb of memory, with approximately 2.6 Gb available (via top)
and a vast swap. What is going on?

Thanks,
Sean