Changing line colors and move text for menu navigation
Why do you need different navXXX and current_navXXX classes? Just have a single

var menus = {//you need some kind of mapping between elements clicked and menu items
  id1: '#home',
  id2: '#design',
  id3: '#seo',
  id4: '#galery',
  id5: '#about',
  id6: '#contact'
};

$('.nav a').click(function () {
    $('.sf_menu li').removeClass('current_nav');
    $(this).addClass("current_nav");

    var selected = menus[$(this).attr('id')];
    $('#content, ' + selected).slideDown(2000);

    for (var menu in menus) {
        if (menus[menu] != selected)
           $(menus[menu]).slideUp(2000);
    }
});