Combobox select change error

Dear All,
 
I want tocreate a selection list itemwhen I am select the gold itemthen show only gold item in the list and when I m select the silver item then it show the only silver item in the list box.
 
I am using following code but it gives a error “conversion faild when converting the nvarchar value ‘Gold’ to data type int.” so what is the problem in my code.

private void cmboItemType()    //for combobox
        {
            string connstr = @"Server=.\SQLEXPRESS ;Initial Catalog=RPSJDB;Integrated Security=True; Max Pool Size=100";
            SqlConnection conn = new SqlConnection(connstr);
            SqlCommand cmd = new SqlCommand("SELECT * FROM ItemTable", conn);
            conn.Open();
            SqlDataReader sdr = cmd.ExecuteReader();
            ArrayList ItemStore = new ArrayList();
            while (sdr.Read())
            {
                ItemStore.Add(new AddValue(sdr.GetString(1), sdr.GetInt32(0)));
            }
            sdr.Close();
            conn.Close();
            cmbItemType.DataSource = ItemStore;
            cmbItemType.DisplayMember = "Display";
            cmbItemType.ValueMember = "Value";
            ItemHaveBeenAdded = true;
        }
        private void lstboxItem()    //for listbox
        {
            string connstr = @"Server=.\SQLEXPRESS ;Initial Catalog=RPSJDB;Integrated Security=True; Max Pool Size=100";
            SqlConnection conn = new SqlConnection(connstr);
            this.lstbItem.Items.Clear();
            SqlCommand cmd = new SqlCommand
                ("SELECT * FROM  ItemTable Where cmbItemType =" + cmbItemType.SelectedValue, conn);
            conn.Open();
            SqlDataReader sdrItem = cmd.ExecuteReader();
            while (sdrItem.Read())
            {
                this.lstbItem.Items.Add(sdrItem.GetString(1));
            }
            sdrItem.Close();
            conn.Close();
        }
        private void cmbItemType_SelectedIndexChanged(object sender, EventArgs e)
        {
              if (this.ItemHaveBeenAdded)
                lstboxItem();
        }
        public class AddValue
        {
            private string Displaym;
            private long Valm;
            public AddValue(string Display, long Value)
            {
                Displaym = Display;
                Valm = Value;
            }
            public string Display
            {
                get { return Displaym; }
            }
            public long Value
            {
                get { return Valm; }
            }
        }