ATI had a demo system running a pair of Radeon X1900s in CrossFire with a third X1900 card dedicated solely to physics processing. This configuration was appropriately referred to as the "meat stack," and while it produced silky frame rates in a number of demos, it's not the only Radeon configuration that will support GPU physics. In addition to supporting three-card configs, ATI will also allow a pair of its graphics cards to split rendering and physics between them.

The TechReport