How many PCI slots of what type are provided by the different Mac Pro models? How do you add another PCI card?

The original Mac Pro Quad 2.66 has four PCIe expansion slots with one double-wide 16-lane PCI Express slot occupied by a NVIDIA GeForce 7300 GT in the default configuration, and three full-length PCI Express slots open.

The Mac Pro "Eight Core" 2.8 (Early 2008) has two PCIe 2.0 and two PCIe expansion slots, with one double-wide 16-lane PCIe 2.0 slot occupied by an ATI Radeon HD 2600 XT in the default configuration. This leaves one 16-lane PCIe 2.0 slot and two four-lane PCIe slots open.

The "Early 2009" Mac Pro models -- the Mac Pro "Quad Core" 2.66 (2009/Nehalem) and Mac Pro "Eight Core" 2.26 (2009/Nehalem) -- have four full-length PCI Express (PCIe) 2.0 expansion slots, two x16 slots and two x4 slots. In the default configuration, one PCIe 2.0 x16 slot is occupied by the graphics card.

To add a PCIe card to the Mac Pro models, site sponsor Other World Computing provides a step-by-step video of the simple procedure. This video specifically covers the "Early 2009" models:

In addition to providing helpful upgrade videos, as you would expect, Other World Computing also sells a variety of PCIe cards as well as other upgrades -- for all of the Mac Pro models.

How does the original Mac Pro allocate lanes for the PCIe bus? What configurations are possible?

The Power Macintosh G5 Quad 2.5 has a four slot PCI Express bus with a fixed total of 32 lanes (with a 16-lane, 4-lane, 8-lane, and 4-lane slot) and the subsequently introduced Mac Pro "Eight Core" 2.8 (Early 2008) has a fixed total of 40 lanes (with a double-wide 16-lane PCIe 2.0 slot, single-wide 16-lane PCIe 2.0 slot, and two 4-lane PCIe slots).

The original Mac Pro Quad 2.66, on the other hand, has a four slot PCI Express bus with a total of 26 dynamically allocated lanes. By default, the graphics card occupies a double-wide 16-lane PCI Express slot, the second slot is allocated as a single lane, and the third and fourth slot are each configured as 4-lane slots.

Apple's PCI Developer's Note provides this helpful diagram of the default configuration:

However, if you install an additional PCI Express card and restart the Mac Pro, MacOS X will provide several options for allocating the lanes.

As provided by Apple's technical document, the remaining lane allocation options are:

	Slot 1	Slot 2	Slot 3	Slot 4
Option 1	8-lanes	8-lanes	1-lane	8-lanes
Option 2	8-lanes	8-lanes	4-lanes	4-lanes
Option 3	16-lanes	1-lane	1-lane	8-lanes

So, if you install a second graphics card in slot four, it would be best to configure the PCIe slots in the 16/1/1/8 configuration to maximize bandwidth. If you need to install a third graphics card or another PCIe card that requires more than a single-lane slot, you will be forced to reconfigure the slots as 8/8/1/8, dropping the amount of available bandwidth to the first graphics card from 16-lanes to 8.