You need to keep in mind that your dedicated Physx card will only at all be useful in games that use GPU accelerated Physx. There are really only a handful of games where this is particularly important. Namely Mafia 2, Batman: AA, Metro 2033, Mirror's Edge and Sacred 2 are the only games where it really matters. Unless you plan on playing these particular games a lot your money would probably be better spent elsewhere. For example selling the GTX 275 and adding in the money you are about to spend on a Physx card can probably get you a GTX 470 or HD5850. That would help you in every game not just ones with GPU accelerated Physx.
Mmmmm i see.... well i think about it. Thanks.