obsidian86 :
you could http://ati.amd.com/technology/crossfire/physics/index.html
That's the 2006 version of HavokFX that never really got past the demo stage, Intel bought it and totally changed the methodology.
Havok phyics now is nothing like it was in 2006, and by 2008 ATi said it was dead.
The current Havok is OpenCL based an is focused on hardware above the X1K series, and likely won't even be code for the HD2K and GF8&9 series.
As for multi-card situations, there's not much detail on it yet, most early trials are 1 GPU.
As for Xfiring HD5K and HD4K, nearly impossible due to the changes in architecture.
It's pretty safe to say no Xfire for different generations, only possible use would be general style computing.