I guess physics based things are mainly for particle collisions, fluids etc., so most of the physics is done by CPU. I'm a AMD fan so I recommend buyin AMD gpu,, it wont affect much in the game. Physx was created by a company called aegsia (not sure the name exactly) which is now bought by nvidia and inculcated in itd cuda technology. but AMD's eyefinity is a better technology. games like crysis, laracroft etc are better played on an AMD GPU.