Pretty interesting video emerges on YouTube today. Batman: Arkham Asylum built-in benchmark was used to demostrate that Crossfire setup (2x HD5870) is actually working along with Nvidia GPU (9800GT), dedicated for PhysX calculations (author was using GenL PhysX mod, I guess).

However, with PhysX effects set to High, fps is bottlenecked with PhysX simulation, as it went from average 301 (with PhysX disabled), to average 125. Still, more than enough to play the game without any inconvenience.