In this paper, we consider an M-ary quadrature amplitude modulation (M-QAM) scheme combined with multiuser diversity over Nakagami-m fading channels. Assuming that delayed but error-free signal-to-noise ratio (SNR) feedback is available, we derive closed-form formulas for the average transmission rate and the average bit error rate (BER), which are also shown to be generalizations of many previous results. Through numerical studies and simulations, we check the validity of our analysis. In addition, we investigate the impact of the Nakagami fading parameter m and feedback delay on system performance.