NVIDIA requires that you use two of the same video card when you use SLI, and even then, the drivers MUST support the application/game in order to properly work.
ATI allows different video cards to work together in Crossfire, as long as they are fairly recent(X1xxx series or above). So you can have an X1800 and an X1300 working together to accelerate an image. Drivers only need to support Crossfire in the first place, but don't require application specific code. Note that at the high end, you DO need at least one master card at the moment, but that requirement will go away in time as Crossfire evolves.
The advantage to the ATI method is that when you upgrade your video card to the latest and greatest, you have the option to use your old card for the physics. With NVIDIA, since you need to have two of the same card to even work together, unless NVIDIA figures out how to link together two different generation cards via their SLI implementation, it may not be possible. So you can't have a geforce 8800(or whatever they call it) combined with a geforce 7800 currently in SLI, and who knows if the physics would let you do it.
On a single GPU machine, having a seperate card to do physics would be better, but nothing would stop you from tossing a low-end video card in as a secondary if it's able to handle the physics demands of the application.
