munin

Install munin and dependencies.
sudo aptitude install munin munin-node munin-plugins-extra libdbd-pg-perl

Let munin make suggestions for you.
sudo munin-node-configure --suggest| grep postgres

The above should reply with several lines similar to this.  read more »

Plugin                     | Used | Suggestions                            
------                     | ---- | -----------  
postgres_bgwriter          | no   | yes                                    
postgres_cache_            | no   | yes (+ALL +gis)