I assume you are actually using JBoss 3.2.x as you have shown content for a *-ds.xml rather than a *-service.xml of JBoss 3.0.x.
Assuming this, if you use the jmx-console web application that is included in the default instance (http://localhost:8080/jmx-console), you can browse and find the JNDI binding for your data source.
You will find, that the JNDI binding, based on your example, is java:/PostgresDS.
So your lookup should be "java:/PostgresDS".
Hope that helps.