scaling up postgres

Hello,

I am setting up a postgres server that will hold a critical event witin
the next few weeks.
It's national exam result (140000 students)
the problem is that the first few hours there will be a huge traffic,
(last year 250K requests only the first hour)
I do have 2 identical beasts (4G - biproc Xeon 3.2 - 2 Gig NIC)
One beast will be apache, and the other will be postgres.
I'm using httperf/autobench for measurments and the best result I can get
is that my system can handle a trafiic of almost 1600 New con/sec.

I cannot scale beyond that value and the funny thing, is that none of the
servers is swapping, or heavy loaded, neither postgres nor apache are
refusing connexions.
my database is only 58M it's a read only DB and will lasts only for a month.
here is my pstgres.conf :
---------------------------- postgres.conf - begin
max_connections = 6000
shared_buffers = 12288
work_mem = 512
maintenance_work_mem = 16384
effective_cache_size = 360448
random_page_cost 2
log_destination 'stderr'
redirect_stderr on
log_min_messages notice
log_error_verbosity default
log_disconnections on
autovacuum off
stats_start_collector off
stats_row_level off
---------------------------- postgres.conf - end
---------------------------- sysctl.conf (postgres) - begin
fs.file-max= 5049800
net.ipv4.ip_local_port_range= 1024 65000
net.ipv4.tcp_keepalive_time= 120
kernel.shmmax= 2147483648
kernel.sem= 250 96000 100 384
---------------------------- sysctl.conf (postgres) - end
kernel semaphores are grown as postgres needs.

vmstat is not showing any "annoyance" I cannot find where is it blocked !

please, I need help as it's a very critical deployment :(

NB : apache when stressed for a static page, i can handle more 16k new con/sec