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.
postgresql.conf:
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: http://web.archive.org/web/20071215213300/http://www.budget-ha.com/postgres/shared-memory.jsp
Much more information about PostgreSQL performance tuning available off the PostgreSQL wiki:
