Setting Shared-Buffers

Hello People,

Today, I've upgraded a dedicated postgres server, from 2 Gb to 10 Gb.
Everything gone well.

But, I would like shared buffers to use at least 5 Gb of the total memory.
Setting kernel.shmmax with 6291456000 (6000 Mb) is not working properly, the
server is changing the value to a small one.
So I can't set the shared buffers to the value that I want.

Now, just momently, the server is running with only 2 Gb of shared buffers,
but I want to use all the capacity of the server/memory.

Can anyone help you,

Linux Fedora Core 9
postgres=# select version();
version
---------------------------------------------------------------------------------------------------
PostgreSQL 8.3.5 on i686-pc-linux-gnu, compiled by GCC gcc (GCC) 4.3.0
20080428 (Red Hat 4.3.0-8)
(1 row)

Thnks,

Rafael Domiciano