A high-capacity memory subsystem architecture utilizes multiple memory modules arranged in one or more clusters, each attached to a respective hub which in turn is attached to a memory controller. Within a cluster, data is interleaved so that each data access command accesses all modules of the cluster. The hub communicates with the memory modules at a lower bus frequency, but the distributing of data among multiple modules enables the cluster to maintain the composite data rate of the memory-controller-to-hub bus. Preferably, the memory system employs buffered memory chips having dual-mode operation, one of which supports a cluster configuration in which data is interleaved and the communications buses operate at reduced bus width and/or reduced bus frequency to match the level of interleaving.