Need help with postrgeSQL

Hi,
can someone help me with configuration of postgresql.conf file. I want to make postgreSQL to load high amount of data in RAM because my HDD is not very fast. I set:
shared_buffers = 10GB ,
work_mem = 256MB ,
maintenance_work_mem = 512MB ,
fsync = off ,
checkpoint_segments = 20 ,
effective_cache_size = 16GB ,
autovacuum = off.
But when I check resource usage when I try to render something it is max 3-4GB of RAM , and rendering time is very slow … any better suggestions for configuration of postgresql.conf file. I have machine with 32GB RAM, 1HDD 2TB 7200rpm, Intel core-i7 - 2600, Ubuntu 10.04 64-bit Lucid, mapnik2 + mod_tile + postgreSQL 8.4 + apache2 installed.