Our mate John Gillooly over at APC Magazine (big Aussie magazine) has posted a quick article looking at dual Radeon X850XT Crossfire performance in 3DMark05, Doom 3 and Half Life 2. They're using an ECS PA1 MVP Extreme motherboard, Pentium 4 3.73GHz Extreme Edition and 1GB of DDR2-667.
First test was 3DMark05. In the standard 1024x768 test the Crossfire configuration came out 40% faster than a single card. However when we increase the load on the graphics subsystem by moving to a 1280x1024 resolution, turning AntiAliasing up to 6x and Anisotropic filtering to 16x we see the Crossfire setup really shine, delivering a score 85% faster than a single card. Moving to the OpenGL based Doom3 we see the Crossfire setup come out 36% faster in testing at 1280x1024 Ultra Quality. In Half-Life 2 we see less of a performance improvement at 1280x1024 with Crossfire coming out 7-10% faster when all details are set to their maximum.