Check out driver issues. NVidia has decided "No PhysX for YOU!" if you have an ATI card installed in your system. You'll have to get older driver versions for the card that will run PhysX; I don't know if such drivers would work with a new card like he GT220, but they should work with a 9500GT or 9600GT.
Considering your main card is only $130-140 devoting $100 to a Physx card seems... a bit wrong. There's only one game were Physx makes any real difference imo and that was mostly a design choice rather than something that was particularly necessary. If physx is really that important to you for some reason you would be better off waiting until the new Nvidia cards come out and selling your HD5750.
What games do you play?
Most probably not worth it, if you skimped & got a cheapie 9400GT/9500GT then your not much better off than using the CPU, if you bought a 9600GT then you should rather spend the $80 on a better main GPU, 5750-->5770?
physx is a nice bonus feature if you have a card that supports it anyway, but in my opinion, is never, EVER worth spending extra money on.
Virtually no games will make it an essential part unless ATI comes out with something compatible, which as far as I know, isn't going to happen any time soon, not to mention it would render this a moot point anyway.