All Mac Pros use a card different from that which is readily available. The card is (supposed to be) only available to certified hardware technicians.
They're required to install the card for you; installation is included in the price of the card.
You don't have to take anything apart, it's really a very simple process, just takes great dexterity to fit the wires onto the card
No, Apple won't sell you the card directly unless you're a technician. You may have more luck with a 3rd party (I got mine from CompUSA after many, many hassles).
I state all of the above from first-hand experience; beware of ignorant AppleCare employees and others who will tell you the card they sell is for all Macs. It's not true–the MP is unique.
See
here for installation instructions