PCI-Express is intended to be used as a local bus only. Due to it being based on the existing PCI system, cards and systems can be converted to PCI-Express by changing the physical layer only – existing systems could be re-booted on PCI-Express and never even know it. The higher speeds on PCI-Express allow it to replace almost all existing internal buses, including AGP and PCI, and Intel envisions a single PCI-Express controller talking to all external devices, as opposed to the current northbridge/southbridge solution in current machines.
PCI-Express is not, however, fast enough to be used as a memory bus. In this respect it is at a distinct disadvantage to the similar HyperTransport which can be used for this role as well. In addition PCI-Express does not offer the flexibility of the InfiniBand system, which has similar performance, but can be used for both internal and external buses.