If you think that sounds ridiculous, you're not alone. Consider:
1. ATI's Radeon X1000-series 3D cards are DirectX 9 parts. By the time any games are out with Havok FX features, Windows Vista, and with it DirectX 10, will either have arrived or will be arriving imminently, rendering the current generation of Radeons obsolete. We're sure ATI will include Havok FX acceleration in its next-gen cards, but you'd be foolish to buy the Radeon X1900 XT card ATI is highlighting now in anticipation of future boosted game physics.
2. ATI's two-card physics acceleration presents the same problem as Nvidia's. By using a 3D card for physics acceleration, you sacrifice 3D performance for physics performance. Yes, there's lots of new technology on the horizon promising faster computing, but if you've played the Elder Scrolls IV: Oblivion, you know that it chokes even the most advanced current-gen hardware. Smooth frame rates are still more important than more flying bricks. As for the three-card option, ostensibly you can plug in an older Radeon alongside a two-card Radeon CrossFire setup to ramp up the physics, but we'll withhold judgment until we know more about pricing of the special three 3D slot motherboards.
In fairness to ATI, it's too early to purchase any hardware with physics in mind. The game developers are still working to include meaningful physics-based content into their games. Until that happens, we have nothing to test with and therefore can't determine whether Ageia's, ATI's, or Nvidia's approach will offer the best value and the best experience improvement. So far, though, we like Ageia's approach the best, both for simplicity of concept and because it doesn't involve sacrificing a GPU.