1. DisplayMember property should be the name of the column to be displayed inside the combobox.
2. ValueMember property should be the name of the column which is the value of the item.
3. If you want the value of the selected item you should use:
cbClient.DataSource = ds.Tables["CLIENTS"];
cbClient.DisplayMember = "NAME";
cbClient.ValueMember = "ID";
If the value is a string than do:
object sSelectedClient = cbClient.SelectedValue;
string sSelectedClient = (string) cbClient.SelectedValue;