It wasn't too long ago that ATI/AMD released the latest set of drivers for their product line. We have come to expect these updates on a very regular basis, but the month of November brought a new twist to the driver set; software CrossFire support. While this was already showing itself as a huge step in the right direction, ATI went a step further and negated the requirement for a CrossFire Master Card. Now it was possible to utilize the CrossFire technology without having to purchase an impossible to find Master Card. There are a few caveats to the new drivers, but it is still a definite improvement in past CrossFire requirements.
In order for the software based CrossFire to work, you need any combination of two CrossFire Ready video cards from either the X1900 family or the X1950 family. The exception to this rule is the X1950 Pro, which uses its own form of interconnect and cannot be used in this method of CrossFire. Also, you have to have a motherboard that is capable of a CrossFire graphics setup. While not all inclusive, many motherboards using the Intel 965/975 series chipsets and the ATI chipsets fall into this category.
Additionally, if you choose two video boards that have a different number of rendering engines, both boards will perform at the lowest common denominator of the two video cards. If clock speeds are different, both boards will run at their respective speed.
For more information, check out ATI and their FAQ.