$(window).addEvent('domready', function(){
        var frameDivs = $$('#frame_preview div');
        var navItems = $$('.nav li');
        // Set style in JS for degradation
        frameDivs.setStyles({
                'position': 'absolute',
                'left': 0,
                'top': 0,
                // Hide on first load
                'opacity': 0
        });

        navItems.addEvent('mouseenter', function(){
                // Fade out any div showing
                frameDivs.fade('out');
                // Fade in div with index equal to this list item
                frameDivs[navItems.indexOf(this)].fade('in');
        });

});