Apple Mac Pro (Early 2008) - Manual (Page 63)

Chapter 3 Making a Good Thing Better 63
6 Align the card’s connector with the expansion slot and press the card into the slot.
Did You Know?—Installing a Full-Length PCI Card
It may be easier to install a PCI card if you remove the hard disk drives and carriers,
and remove adjacent cards. Make sure the end of the card goes in the appropriate
card guide before aligning the card with the PCI slot and completing the installation.
Port access opening
PCI Express slot
PCI bracket
Brace
Card guides