What exactly is your expectation from a monitoring system? If it is just a postgres server that you want to monitor and and regular alerts, nagios is the shortest path. But if you want more, for example enterprise level monitoring system (graphs, history, maps, predictions) and if you have many other servers along with postgres server(s), I personally suggest you zabbix, but zabbix needs some additional helpers like mamonsu or libzbxpgsql for for postgres monitoring.
