Does the Mac Pro support standard PC video cards? Does it support SLI or Crossfire?

Video cards require specific drivers and firmware to work with the Mac Pro models, so "standard" PC video cards are not supported.

For Mac users who may not yet be familiar with the terms "SLI" and "Crossfire" from the Windows world, "SLI" stands for "Scalable Link Interface" and NVIDIA defines the technology as:

A revolutionary platform innovation that allows you to intelligently scale graphics performance by combining multiple NVIDIA graphics solutions in a single system with an NVIDIA nForce SLI media and communications processor (MCP).

SLI technology is the only multi-GPU solution that was built from the ground up with dedicated scalability logic in each GPU. This logic enables the most efficient communication between GPUs providing the best scalability performance. SLI technology also has the most comprehensive list of supported games.

ATI explains that the rival Crossfire technology is different because:

CrossFire can enable multi-GPU rendering on all applications.

CrossFire supports more rendering modes. Supertiling evenly distributes the workload between the two GPUs to improve performance. CrossFire can use multiple GPUs to improve image quality rather than performance with Super antialiasing (AA) modes. Supertiling and SuperAA modes are only supported on the CrossFire platform.

CrossFire is an open platform that supports multiple components and graphics cards that can be mixed and matched in a single system. Competitive multi-GPU solutions are constrained to supporting identical graphics cards.

In basic terms, SLI and Crossfire both make it possible to directly connect multiple graphics cards for improved performance. The computer must have multiple PCI Express slots, and for a high-end configuration, the computer must have two 16-lane slots.

The original Mac Pro Quad 2.66 dynamically allocates lanes, and cannot be configured with two 16-lane slots. It can, however, be configured with two 8-lane slots. The Mac Pro "Eight Core" 2.8 (Early 2008), on the other hand, has two fixed 16-lane PCIe 2.0 slots.

Consequently, the original Mac Pro theoretically could work with 8-lane SLI or Crossfire configurations, and the "Early 2008" Mac Pro could work with 16-lane SLI or Crossfire configurations. However, as of the date last updated (see top), neither NVIDIA nor ATI have announced plans to provide SLI or Crossfire support for Mac Pro models.