In this paper, the uncoded symbol error rate (SER) versus signal to noise ratio (SNR) curves of M-ary quadrature amplitude modulation (M-QAM) multiple-input multiple-output (MIMO) systems using zero-forcing (ZF) or maximum likelihood (ML) detection employing various numbers of antennas are given analytically and empirically. Using the given formulas, the designer of a MIMO system can easily analyse the tradeoff between using ZF or MLD, where ZF is computationally efficient but requires more receivers to achieve the same performance as MLD. An example is given, showing that, for an uncoded MIMO system using QPSK, 16QAM, or 64QAM, to achieve the same or better performance as MLD using 2, 3, and 4 antennas at each end, ZF requires at least 3, 5, and 7 receivers, respectively, at a specified average SER of 10-3.