AMD Zone has just posted a dual graphics card showdown matching CrossFire vs SLI vs 16X SLI. They compare using the X850XT PE in CrossFire mode versus SLI 7800GTs on the nForce 4 SLI, and the new nForce 4 16X SLI. They look for performance differences between the SLI setups, and against CrossFire.
They also investigate CrossFire modes looking at performance for AFR mode versus the default modes of Scissor and Supertiling. To do this they use the following OpenGL and Direct3D benchmarks. 3DMark2K5, Doom 3, Quake 4, F.E.A.R., FarCry, and Half Life 2. They are particularly interested in which games support the faster AFR mode, and what happens to performance when they do not.