[jQuery] Slide down and up element and remain vertical size

[jQuery] Slide down and up element and remain vertical size


I have an example here:
http://www.mech7.net/tmp/slide_down/
When i hover the elements it jumps a little in vertical space, does
anybody know a solution how to remain the space my code is :
$(document).ready(function(){
    $('#portfolio div.item:first > div').addClass('open');
    $('#portfolio div.item:first > h3').addClass('active');
    function slideDownEffect(){
        var speed = 100;
        $('#portfolio div.item').mouseenter(function () {
            if($(this).children('h3').hasClass('active') == false)
{
            $('#portfolio div.item div').slideUp(speed);
            $(this).children('div').slideDown(speed, slideDownEffect);
            $("#portfolio div.item").unbind('mouseenter');
            $('#portfolio div.item h3').removeClass('active');
            $(this).children('h3').addClass('active');
            }
        });
    }
    slideDownEffect();
});