I did not come to this conclusion only by myself. It came out with a discussion with one of the MVP (SQL Server) Vinod Kumar on a recent Tech-Ed session. I have explained it in the previous post.
Hey, even I have heard about this rule.
This is NOT a hard and fast rule, what it means is - If you have a RAM more than your DB size then increasing the RAM may not help!
Do you also want to know how many DB's I have worked on?