//plugin  
jQuery.fn.topLink = function(settings) {  
    settings = jQuery.extend({  
        min: 1,  
        fadeSpeed: 200  
    }, settings);  
    return this.each(function() {  
        //listen for scroll  
        var el = $(this);  
        el.hide(); //in case the user forgot  
        $(window).scroll(function() {  
            if($(window).scrollTop() >= settings.min)  
            {  
                el.fadeIn(settings.fadeSpeed);  
            }  
            else  
            {  
                el.fadeOut(settings.fadeSpeed);  
            }  
        });  
    });  
};  

//usage w/ smoothscroll  
$(document).ready(function() {  
    //set the link  
    $('#top-link').topLink({  
        min: 400,  
        fadeSpeed: 500  
    });  
    //smoothscroll  
    $('#top-link').click(function(e) {  
        e.preventDefault();  
        $.scrollTo(0,300);  
    });  
});