The optimal transmit strategies of single-user multi-antenna systems with respect to average capacity maximization are well understood. However, the performance measure does neglect delay aspects which are important for higher layer design. Therefore, we consider the maximization of the effective capacity in a single-user multi-antenna system with covariance knowledge. The optimal transmit strategy is derived and the properties as a function of the decay-rate requirement of the buffer occupancy are analyzed. In particular, we show that the larger the decay-rate requirement, the smaller the beamforming optimality range, i.e., the more spatial eigenmodes are activated. This behavior is illustrated by numerical simulations and explained by the channel hardening effect.