r.a.d.combobox supports automatic sorting (Ascending, Descending) based on the Text property of the control. Just make sure you set the Sort property of the RadComboBox instance to any value of the RadComboBoxSort enum. Valid values are: Asc, Desc, None.
Example:

switch (SortOrder.SelectedItem.Value)
{
case "Ascending" : RadComboBox1.Sort = RadComboBoxSort.Ascending; break;
case "Descending" : RadComboBox1.Sort = RadComboBoxSort.Descending; break;
default: RadComboBox1.Sort = RadComboBoxSort.None; break;
}