The WS-X6408A-GBIC module offers the same features as those offered in WS-X6408-GBIC. In addition, the WS-X6408A-GBIC module offers Enhanced Quality of Services(QoS) by providing one additional receive queue and one additional transmit queue per port, and supports WRED (Weighted Random Early Detection) for congestion avoidance.
You can use the 6408A card inplace of 6408 card.