Trouble getting effective_cache_size parameter to take

I've got a Postgres v8.2.4 server that we recently upped from 2GB RAM to
6GB. I added:

effective_cache_size = 5120MB

to the postgresql.conf file but when I restart the server and do a "show
all" it always comes back with "1000MB". I can set it manually and it
takes but for some reason it just seems to ignore it in the config file.

Is there something else that needs to be present for this? I've tried
setting it in both the numeric-only and "MB" forms. I've included a
portion of the config at the end here, I realize it's probably not quite
right yet.

Thanks,
Joe

#-----------------------------------------------------------------------
----
# RESOURCE USAGE (except WAL)
#-----------------------------------------------------------------------
----

# - Memory -
shared_buffers = 600MB
work_mem = 100MB
maintenance_work_mem = 300MB
max_stack_depth = 5MB
effective_cache_size = 5120MB

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