physx is just a name....there isnt a huge performance boost or anything. Its like ATI's havok. Just check the games you want to play and see the framerates for that card with that game. The game i know of that is brand friendly is Crysis with Nvidia and maybe Farcry 2 with Nvidia but im not sure since the ATI 4870x2 gets amazing FPS on that game and matches the gtx 295.
All in all, just read reviews of the cards you have in mind and look at FPS charts of games and see which one matches what your looking for.
http://www.tomshardware.com/charts/...009/Sum-of-FPS-Benchmarks-1680x1050,1197.html
there thats the FPS chart im talking about, take a look at it and the specific games you will be playing.