For Phsyx all you need is a 460 1GB.
But why get it when 3 games support it. Are you a batman freak ? Or Dirt 3 freak ?
Other then that its a waiste of money, a waiste of space and heat making your actual video card get hotter.
Also now with powerful nvidia cards, you can just use its own phsyx and you dont need a dedicated card. Also with powerful CPU's you can have your CPU do the phsyx. but usually leave it on default which is auto select. What you scared your not getting the phsyx you should in batman ? Or Borderlands 2 or whatever game you playin? It was good years ago when GPUs were slower, but now,,,,, phsyx on a 6xx can be used no need for extra card. You will see no difference except your case will get hotter and your GPU will get hotter. That is all it will do for you. UE4 have phsyx ? Cry Engine 3 have phsyx ? Your putting too much emphasis on phsyx,, its a useless tech that is in 1 out of every 20 games. forget it. Enjoy your graphics card dont cram another one making yours hotter for no reason and use extra power wattage. gl