Most FireWire busses provide 400 Mbit of bandwidth, though products with 800 Mbit bandwidth have been available for some time. When connecting FireWire devices that operate at different speeds, the slowest unit in the FireWire network determines the speed of all devices in a network. With most Apple Macs, the 800 Mbit FireWire port can be used with 400 Mbit devices, but the connectors are different and a FireWire 800-to-400 adapter will be required.
As a general rule, the FireWire bus is a subset of the available PCI / PCIe bus bandwith, which is 1 GigaBit. This will mean that FireWire and PCI are sharing the same available resources. On most Apple Macs the FireWire bus is independent of the PCI / PCIe bus (This is also the case with some 1 gHz machines, but only a few of them!).
Please verify with the manufacturer for details.