PCI Express is a very powerful technology with many of strong features that make it suitable for high-performance, chip-to-chip interconnect needs. The overall performance of the systems depends how quickly the data flows from node to node, and also how efficiently the data flows from end to end. The replay mechanism in PCI Express helps to boost the end-to-end performance by being able to resend packets locally between nodes. This paper describes the conditions that affect performance in PCI Express and the special role of the replay buffer in achieving high performance The paper also explores how to decide the appropriate size of the replay buffers for particular needs.