Dynamic resource allocation combined with adaptive antenna arrays at both the transmitter and receiver can achieve further QoS improvement in conventional OFDM systems. In this paper, we propose two different dynamic resource allocation algorithms with adaptive beamforming for MIMO/OFDM systems and investigate their performance over multipath fading channels under perfect and imperfect CSI. These proposed approaches involve the use of adaptive modulation and adaptive frequency-domain power allocation. By employing this proposed methodology, significant performance improvement can be achieved as compared to the conventional adaptive antenna arrays based on OFDM systems. It is shown, however, that the adaptive modulation based approach is too sensitive to channel estimation errors and that its performance is worse than the adaptive frequency-domain power allocation approach under imperfect CSI.