I have seen some cheepo cameras only play back the tapes that they recorded, most likely this is because the cheeper heads become misaligned more easily.
I would say take to cameras to the shoot. One camera doing more of a wide shot, and the other camera doing more of a close up. This way you can always cut to the other.
You did not answer all the questions I asked. What is the make and model of the fire wire card, it is possible that it is not compatible with macs or with your model of mac.