JQuery slide effects on drop down menu

I created jquery dropdown menu like this. I want to add slide effect on drop down menu. At the same time if I am placing my mouse over my menu tab the sub menu was opened now it is looking like one step up compare to the menu tab. I need slide like this. I here added my script.

function mainmenu() {
    jQuery(" #na1").hover(function () {
//alert("hai-2");
        jQuery(this).find('#content1').slideDown("fast");
    }, function () {
        jQuery(this).find('#content1').slideUp("fast");
    });
}

$(document).ready(function () {
    mainmenu();
});

$(document).ready(function () {
    $("#na ul li").hover(function () {
         $(this).siblings().find('ul').slideUp(400);
         $(this).find('ul').slideDown(400);        
    }, function () {
       $(this).find('ul').slideUp(400);
    });
});

Demo:

http://jsfiddle.net/QkbDg/1/