If you try to apply the Show-Hide behavior on the top layer it will flicker like crazy. To fix that do the following:
Here is the code:
<div id="topLayer" onmouseover="MM_showHideLayers('topLayer','','hide')">Message</div>
<div id="bottomLayer" onmouseout="MM_showHideLayers('topLayer','','show')"></div>
You may have to add an image to your bottom layer. It would be a transparent .gif the same size as the layer.