Ageia vs Havok
 

Ageia have released a range of physics chips called PhysX that improve explosions / collisions, etc in games. Havok are doing something similar where you can allocate part of the GPU time to physics if you have two graphics cards in SLI mode.

Have you tried either of them? Only a few games support them at the moment - such as Ghost Recon Advanced Warfighter and Cell Factor.

I haven't tried Havok yet (although I do have two graphics cards), but Cell Factor has some nice 'physx kill' options in it with an Ageia card.