Upper limit on number of buffers?

This line works:
/usr/local/pgsql/bin/postmaster -N 32 -B 928 -i -S
-D/home/postgres/pgdev -o "-F -fs -S 4096"

Where as this line:

/usr/local/pgsql/bin/postmaster -N 32 -B 1024 -i -S
-D/home/postgres/pgdev -o "-F -fs -S 4096"

does not.

Any ideas? 
I have 256M of memory, RedHat Linux 7.0, CVS version of Postgres as of a
couple days ago.

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