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) -- and the "Mid-2010" Mac Pro models -- the Mac Pro "Quad Core" 2.8 (2010/Nehalem) and "Eight Core" 2.4 (2010/Westmere) -- all 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. There is a 300W combined maximum for all PCI Express slots in these models as well.

To add a PCIe card to the Mac Pro models, site sponsor Other World Computing provides helpful step-by-step videos of the simple procedure.

This video specifically covers the original and "Early 2008" models:

Although the procedure is very similar, this video is for the "Early 2009" and "Mid-2010" 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.

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