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

[Mech7]

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();
});