Problem with database performance, Debian 4gb ram ?

Hi Hi all,

I have now readed many many forums and tried many different solutions and I
am not getting good performance to database. My server is Debian linux, with
4gb ram, there is also java application and I am giving to that 512mb
(JAVA_OPTS) memory. In database there is now like 4milj rows. What should I
do better.
Now .conf is:

max_connections = 80
shared_buffers = 512MB
temp_buffers = 8MB
work_mem = 20MB
maintenance_work_mem = 384MB
wal_buffers = 8MB
checkpoint_segments = 128MB
effective_cache_size = 2304MB
cpu_tuple_cost = 0.0030
cpu_index_tuple_cost = 0.0010
cpu_operator_cost = 0.0005
fsync = off
checkpoint_timeout = 1h

and I am giving kernels like:

sysctl -w kernel.shmmax=1073741824
sysctl -w kernel.shmall=2097152

btw, what file I should modify to give this kernels as defaults ?

Thank you very much ! Hope you can clear my problem !