hot restart of posgtresql

hi all
by unknown reason our database server rebooted couple hours ago.
now i cannot restart posgresql 8.1
heres the error code i get:

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root(at)eddie:/var/run# /etc/init.d/postgresql-8.1 start
Starting PostgreSQL 8.1 database server: mainThe PostgreSQL server failed
to start. Please check the log output:
2007-07-03 19:20:42 EDT 2928 FATAL: could not create shared memory
segment: Invalid argument
2007-07-03 19:20:42 EDT 2928 DETAIL: Failed system call was
shmget(key=5432001, size=3276259328, 03600).
2007-07-03 19:20:42 EDT 2928 HINT: This error usually means that
PostgreSQL's request for a shared memory segment exceeded your kernel's
SHMMAX parameter. You can either reduce the request size or reconfigure
the kernel with larger SHMMAX. To reduce the request size (currently
3276259328 bytes), reduce PostgreSQL's shared_buffers parameter (currently
393216) and/or its max_connections parameter (currently 250).
If the request size is already small, it's possible that it is
less than your kernel's SHMMIN parameter, in which case raising
the request size or reconfiguring SHMMIN is called for.
The PostgreSQL documentation contains more information about
shared memory configuration.
failed!
=======================================================

heres my sysctl:
kernel.shmmax=4294967296
kernel.shmall=536870912

Any help will be really appreciated
Thanks