Well it's definitely worth 1/2 the price of a 285, not sure if that GTX285 price or future prices will change anytime soon, usually we talk about what is the case or will happen in the US (even if some of us aren't from the home of the WOPR ).
From the sounds of things it's a good deal for you.
As for physics, it's still evolving, remember originally nVidia was in the Havok camp along with ATi, showing Havok FX as 'SLi Phsyics', then Ageia entered the scene with alot of promise and some interesting tech demos, but didn't go far enough; nV bought them and took them a bit further, but it's still essentially debris physics (like Dynamic Paper) not the game physics everyone had hoped for half a decade ago when they talk about it in the Brook presentations.
As we go forward with a standardized platforms in OpenCL & Compute Shader standards. It's likely eventually the PhysX arm of nV will chose one of those and port the current CUDA version to one or the other, or else it will die like Glide.
Anyways, at that price, you should really worry about PhysX, and I doubt that there will be a cheap Fermi based solution coming near you anytime soon, they're going to have trouble getting them to the centre of the universe let alone the far reaching regions of our planet.
So buy the card and enjoy the improved gameplay over your aging GTS-320, which was a great value, but is starting to show its age now.