If you go USB put it on the root hub with no other devices. It needs a lot of bandwith, so give it to it.
If you go firewire, you limit your options, but no problems with the bandwith. You just have to live with the fact that you are using a mac proprietary connection.
If you go PCI then you have the most choices, but you might want to glue the card in so that it doesn't rattle loose while you are driving.
To get best results get one with hardware processing. Those are the pricy ones. The cheapo ones are like parasites on your CPU and you don't have much cpu power to begin with. Those cheapos do software processing.
So which connection format do you want/need, and which do you not want/can't have?