I passed the Java 6 certification mainly using Java 5 material, plus some Java 1.4 material. No question that you're going to have to be comfortable with Java 5. For Java 6, read the Sun Javadoc introduction to the java.io.Console class, note things about synchronized methods, single instance per JVM, what methods return String and what returns an array.
Study the java.util.NaviagableSet and java.util.NaviagableMap interfaces. Note what happens with the head() and tail() methods and note the pass-through limitations and what happens if you try to add to the collections returned by these methods.
A few extra hours studying these should help a lot.