It all depends on how u apply the physx. U can run ur physx on ur pc using the cpu to do the work, so it appears nvidia are perhaps just compiling it's structure for different cpus.
Thing being with the application. If b4 hand u know the bounds of which u can work in to produce a decent game, including physx, as long as ur clever u can create tremendous things.
It doesn't apply that u use physx in a game, then the game runs slower. If u make a football game and merely use the motion of the ball as a physx routine, u'll work well. If u include the wind movement of all the players' shirts/hair/etc, then ur stacking it up.
Subsequently, the power of whichever of the cpus inside the wii they use for this, I'd trust the devs on the nintendo side of things to have it all covered. Some really clever people there too as we've found over the years. Wii, for what it's worth (being the lesser power-packing console, a gamecube on steroids) is surpassing sales figures in the console market still, according to this weekends news.
It's great news physx is spreading it's range so much.