In this paper, we consider a decode-and-forward wireless relay network consisting of a source node, a destination node and multiple relay nodes under Rayleigh fading. Cooperative diversity of the network is achieved by using an opportunistic relay selection. To maximize the benefit of cooperative diversity, we propose a novel cross-layer single relay selection scheme, called the First-Channel-and-Second-Buffer (FCSB) scheme, that considers the performances of both the physical and data link layers. To examine the performance of the proposed FCSB scheme, we develop an analytical model that describes the queueing process of the source node, and derive the average packet delay of the FCSB scheme from our analytical model. Our analytical model is verified through simulation. We discuss how to optimally design the proposed FCSB scheme to minimize the average packet delay. We also show that the optimized FCSB scheme outperforms other relay selection schemes from the viewpoint of average packet delay.