Why oh why do people still pay the Apple premium on accessories and peripherals when there's tons of information on affordable compatible hardware?
And stated above, just use a 9-pin to 6-pin FireWire cable to connect FW400 devices to FW800 ports.
The OP mentioned he needed dedicated bandwidth for one of his devices, hence an expansion card. A lot of pro audio gear is like that, actually. Also, I've dealt with older Canon video cameras that for whatever reason (I cannot explain) refuse to work on FW800 ports with adapters.