What do you mean by saying that RadMenu displays behind the page? Is this some other control? By the way you can check this helpt topic which show the default z-indices of RadControls.
I think in your case the container element of RadMenu has lower z-index than the container html element of the other control which RadMenu displays underneath:
<telerik:RadMenu style="z-index:10000000" />
RadMenu will never appear on top of this content
No matter how high the z-index of RadMenu is it will not display on top of the other element. This is actually not specific to RadMenu but to HTML and CSS.
The EnableOverlay property will not help in this case. Overlay is enabled by default and is useful only in Internet Explorer where HTML elements cannot appear on top of windowed controls (dropdown lists, flash etc). Those elements do not obey to z-index stacking rules in IE so a special approach is required. The EnableOverlay was introduced for the rare cases when it was causing side effects. As for the naming convention - you are right - those properties should have been named the same way.
In my opinion RadMenu cannot be directly compared to pure DHTML menu because the latter lacks any server API such as databinding, programmatic creation, AJAX support, server-side events etc.
I hope this helps,
the Telerik team