postgres optimization

hi

i am optmizing postgres database and need some help on it.
I currently have a server with 2 Gb RAM.

I have setup the /proc/sys/kernel/shmall and /proc/sys/kernel/shmmax to
536870912 i.e 512 Mb.
should i change it to 2 Gb and how will it affect the perfomance and will
there be any downsides to it or is it better to keep it less.

I then changed the shared buffers in config file according to this
expression specifies in postgres manual

max connections=128
250kB + 8.2 kB * shared_buffers + 14.2 kB * max_connections or infinity

shared buffer=65277

what value should i specify for Effective Cache Size. I came across this
article but it is slightly confusing.

"Say there is 1.5GB RAM in your machine, shared buffers are set to 32MB and
effective cache size is set to 800MB. So if a query needs 700MB of data set,
PostgreSQL would estimate that all the data required should be available in
memory and would opt for more aggressive plan in terms of optimization,
involving heavier index usage and merge joins. But if effective cache is set
to only 200MB, the query planner is liable to opt for the more I/O efficient
sequential scan."

thanks,
kailash