In broadband wireless OFDMA systems where a set of subcarriers is shared among multiple users, the overall system throughput can be improved by a band AMC mode that selects subbands preferential to individual users. As long as channel qualities for the subbands of all users are known a priori, multiuser and frequency diversity gains can be simultaneously achieved with an appropriate scheduling algorithm. In this paper, an analytical means of evaluating the maximum system throughput that can be achieved with a band-AMC mode is presented under the various system parameters. In particular, practical features of resource management for the OFDMA system are carefully modeled within the analytical framework. Also, we validate the proposed analytical model through a simulation. The numerical results demonstrate that the band AMC mode outperforms the diversity mode as long as sufficient feedback information for channel quality indication is provided for the subbands of all users.