Begin by disconnecting your Mac Pro's power cable and any display monitors currently connected to the back of your Mac Pro. Remove the side panel of the computer. The inside of your computer should look like Figure 1, with the gray fan on the left and your current graphics card on the right.
In Figure 2 you will see a gray button located on the Mac Pro's fan (A). Press this button and slide the fan to the left. This will cause the metal bar (B) that holds the graphics card in place to also slide to the left. Make note of the two small black clips (C) to the right of the fan, they will be used momentarily.
Loosen the thumbscrew (D) and the matching one above it. Remove the retaining plate that they were holding. Remove the bottom-most PCIe cover (E). This is necessary to accommodate the larger fan on your new graphics card. You may now remove your current graphics card by gently rocking it back and forth while pulling it towards you out of the Mac Pro.
Refer back to the black clips mentioned earlier (Figure 2). If your GPU came with power cables this is where you will connect them (some smaller cards do not require and thus do no come with power cables). Insert the small black end of each power cable into the clip until they "snap" into place.
*If you are installing a card that requires an additional power supply please look here for assistance.
Before inserting your new graphics card take note of the bottom-most PCIe slot (F). Just like your old graphics card, your new graphics card has a connector tab along its edge that will fit in this slot. Make sure to line up the connector tab and the PCIe slot correctly and apply gentle pressure until the card "snaps" into place. This should not involve excessive force. If the card does not seem to be inserting correctly, remove it and check the alignment again.
Insert the other end of the power cables into your graphics card, as shown in Figure 6. Installation complete! Begin reassembly by sliding the Mac Pro's fan to its original position. This should slide the metal bar to the right, locking your graphics card in place. Replace the retaining plate and tighten the thumb screws. Replace the Mac Pro's side panel and connect the power cable and your display monitors. Turn on your computer and enjoy!