Assuming balanced computation, the scalability of an iterative reconstruction algorithm on a cluster computer will be determined by the tradeoff between shorter computation times on larger clusters versus longer communication times. To investigate the impact of improving communication performance, we assembled a cluster equipped with dual communication networks: Gigabit Ethernet and PCI-X Infiniband. The cluster consisted of 8 compute nodes and one fileserver node; each node had dual 3.6 GHz Intel Xeon processors and the two dual-ported communication networks. For image synchronization alone, PCI-X Infiniband was 3.8 times faster than Gigabit Ethernet. We benchmarked two parallel OSEM-3D reconstruction algorithms representing a range of image and sinogram sizes, for both a brain-only scanner (HRRT) and a whole-body scanner (HIREZ). In every case PCI-X Infiniband provided significantly superior reconstruction performance at all cluster sizes, and in all cases but one extended the range of cluster sizes over which we observed performance improvements.