This tells you how to specify the JRE with 1.6 (and later 1.5 versions I think)
http://java.sun.com/javase/6/webnotes/family-clsid.html
plus look at these
http://ww2.cs.fsu.edu/~steele/XHTML/appletObject.html
http://java.sun.com/j2se/1.5.0/docs/guide/plugin/developer_guide/version.html
http://java.sun.com/j2se/1.5.0/docs/guide/plugin/index.html
but be prepared for some fun :)