Well, the information on that linked page didn't help me as much as I'd expected. I'll be happy to provide you a screenshot of my Mail and News Preferences for Reply & Forward.
I can assure you that I've checked the box for "Include entire message in reply". I've had that checked for a long time, but it isn't honored if any text is highlighted on the original email (as I'm previewing it) when I click Reply.
Additionally, I would like to use this option:
If you wish for your reply to be on top but your default signature to remain on the bottom, select 'Use Internet-style attribution lines' with 'Place insertion point before quoted text' checked.
...but my version of Entourage has no reference to "internet style," and I can't see that I have an option to choose that.