I have just bought a Mac Pro primarily for editing on the fcp. I shoot most of my footage on the canon 5d mark ii. I have a couple of questions
>> Do I need to purchase a blackmagic card? how does is help? does it help in improving the output?
>> If i have to purchase an external hard disk, how much difference does a firewire hard disk make in comparison to a usb one?
A blackmagic card is a video i/o card. There are also similar cards made by AJA and also interfaces like the Matrox and the Kona iO
There all broadly perform the same function which is to provide a broadcast quality video and audio input and output via standard BNC and XLR connectors.
Typically this would be used with a video monitor and also to provide signals for video scopes, deck inputs and outputs, speakers PPMs etc etc.
Some also perform SD to HD and HD to SD real time conversions.
So you need a Kona or Blackmagic card if you want to have any of that kind of monitoring or deck connection.
Obviously if you use a 5D then you are likely using log and transfer to transode your footage to ProRes rather then tape capture so card is not directly needed. However it would be useful for proper moonitoring.
You will need external drives for your media - if you try saving HD footage to your sytem drive then you will have all kinds of issues. In terms of drives USB is going to be too slow. You will have lots of problems with dropped frames.
Firewire 800 can just about manage ProRes footage but may struggle with the extra bandwidth needed for HQ, certainly more than a single stream.
You should really be looking towards a raid of some kind to give you the drive throughput needed for HD pictures.