In wireless relay networks, the use of multiple antennas at both the source and destination can increase the capacity linearly with the number of antennas. In this paper, we investigate amplify-and-forward dual-hop MIMO relay networks without channel state information at the relay nodes, i.e., noncoherent case. In contrast to the conventional approach where the available power is equally distributed over all relay nodes, we propose an opportunistic relaying approach in which only a few very important relays (VIR) are selected to share the power. By doing so, the array gain of MIMO relay networks can be enhanced and thus maximize the ergodic capacity. Simulation results show that the optimal number of selected VIR to maximize the ergodic capacity is identical to the number of antennas employed at the source or destination. Compared to the conventional approach, the proposed opportunistic relaying approach can achieve a higher capacity.