# basics
sudo port install postgresql8 +server 
sudo port install rb-postgres

# auto-launching
sudo nano -w /opt/local/etc/LaunchDaemons/org.darwinports.postgresql8/postgresql8.wrapper
(in the init section add where your db is, like POSTGRESQL8DATA=/Database)

sudo launchctl load -w /Library/LaunchDaemons/org.darwinports.postgresql8.pl

# create db:
sudo mkdir -p /Database
sudo chown postgres8:postgres /Database
sudo su postgres8 -c '/opt/local/lib/pgsql8/bin/initdb -D /Database --locale=en_US.UTF-8 --encoding=UNICODE'