In many situations, the best way to speed up a stock Postgres install is to increase shared memory buffers.
Shared buffers are counted in 8k blocks. Below is a configuration where Postgres can use (28672*8) = 229376k.

shared_buffers = 28672   # 2*max_connections, min 16

In Linux, kernel shared memory will need to be expanded to handle more than (usually) 32M.

This page has more details and methods:

Much more information about PostgreSQL performance tuning available off the PostgreSQL wiki: