Hi.
I made this customization few months ago for another client here on forum also in this category.
Before you will start to search it .... I will give you few tips which I still remember.
You have to publish menu on "bannertop" position - now you have to possibilities - use default menu module and make some css customization or use advanced menu from JED.
But remember that if you want to have fully customization you have to delete/hide "old" menu code from layout/default.php