I don't know to do in CSS like hiding the "dropdown menu" and then make it visible when a user holds over the menu. But I guess there is some hover property and a visible property or something
Look at my page:
http://stud3.tuwien.ac.at/~e0226430/
The menu is entirely CSS-driven. However, this is not supported in IE. The practical approach is to use this technique and back it up with JS to make it work in IE and look better (sometimes) in other browsers.

But I like the alpha blended menu as it is now. Not sure if that can be done in CSS without a picture.
It can. The opacity property does exactly that.
However, opacity is supported only since Moz 1.6, before that it was -moz-opacity. And IE doesn't support it at all, it supports some other technique to achieve the same effect though.
Don't know about Opera.