Re: GT220 as dedicated phys-x card = bad idea..
You have to understand how fluidmark works. It's not GPU intense at all at rendering the scene, just calculating the physx. So it's only natural that a 470 gets higher performance than a lowly GT 220. Try async mode this will put more stress on the GPU doing the graphics rendering because it will render graphics as fast as it can (higher GPU usage) and the physx framerate will be separate. Also try setting the number of particles to be lower, very few applications use 60,000. I believe the old version used 20,000 or so. Most physx titles use less than 10,000.
This is also proof that a better processor will help out just as much as a high end nvidia card. My GT 240 gets nearly 40 fps in that benchmark with a stock core i7 930 but you get 30 fps with an overclocked Q9650 and a 470 which is around what I got when I had a Core 2 Quad. So your processor is probably limiting how much the 470 can be utilized for physx (check your GPU usage in GPU-Z or another program). Physx seems to really like Core i7 for some reason. I'm not sure why exactly, I know they are good processors but it seems like they aren't THAT much better.
The best test is to try some actual games that use physx like Mirror's Edge and Arkham Asylum (for mirror's edge you will need to overwrite the physx dlls in the game folder with newer ones from the latest physx driver or performance will really suck). The Mafia 2 demo benchmark is also a pretty good test though the framerate will likely suck no matter what you use. But I would be interested if a 470 + 220 performs better than just a 470. I bet it will because Mafia 2 is very graphic intense.