There did used to be 'dedicated PhysX' cards made by Agiea but this was bought up by Nvidia. PhysX is designed to handle the physics used in games (this is how flags look convincing or barrels roll and balls bounce as they should). This was a process normally handled by the CPU but to further performance PhysX was born.
PhysX - Wikipedia, the free encyclopedia
If you own an Nvidia card then it's already PhysX capable as this was introduced from the 8800 series onwards.
Not all games are PhysX capable though and much like DX10 they have to have this specifically written in.. (otherwise the physics is handled by the cpu).
The introduction of DX11 may spell the death knell for PhysX as it handles physics on the GPU much like PhysX.
ATI does not use PhysX although to get round this some users have been using say a 5870 and then having a small nvidia card to handle the PhysX.
SLI or Crossfire is just the name for using multiple card set ups. SLI is Nvidia and Crossfire is ATI. Now to answer your question, if you had a SLI Nvidia set up then you wouldn't need to add a PhysX card as it's already contained on the nvidia cards. If running a crossfire set up however then as i said above a small nvidia card could be added (if you have the slots for it of course) to handle PhysX.
IMO PhysX isn't widespread enough to really worry about it. If you've got it then use it but if not it's no biggie..
Just to add that when pairing Nvidia cards with ATI one has to be careful which nvidia driver is used as nvidia has written in a little bug that if a ATI card is detected then it will stop working (workarounds are available tho')
