How many PCI slots of what type are provided by the Mac Pro?

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.

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.