Ever since its release, Doom 3 performance has been somewhat of a feather in NVIDIA's cap. Their high-end cards simply dominate ATI's current generation with regard to Doom 3 frame rates, as is evident in the graphs above. A single GeForce 7800 GTX is faster than a pair of X850 XT cards running in CrossFire mode, not to mention the 7800 GTX SLI and GeForce 6800 Ultra SLI configurations which are faster as well.
If we focus only on CrossFire's impact on performance, however, things are not all bad. At a resolution of 1280x1024, enabling CrossFire results in performance increases of 65% (No AA) and 79% (4x / 8x). And Doom 3's performance scaled even more at the higher resolution. With Doom 3 running at 1600x1200, enabling CrossFire boosted Doom 3's performance by 76% (No AA) and 81% (4x / 8x).