I went from an Nvidia card to a Radeon without any complications. Just uninstalled the Nvidia drivers/console and had the Radeon stuff already downloaded and ready to install once the new card was in.
Your mileage may vary.
PhysX is a matter of personal choice. Conventional wisdom seems to be that most games don't utilize it with any success; the primary exception being the Batman games where PhysX makes his cape hella cooler than without it. I wouldn't base a card choice off PhysX but then I never really got into the Batman games either
FYI that same card is available at the same price point from Newegg except Newegg has the game bundle showing with it. Also, in both those cases the $159 is after rebate which will be in the form of an American Express gift card. Just so you know. I'm always hesitant to count rebates against my purchase price for just that reason -- you can always use it to buy groceries or pay for dinner but it's a lot harder to pay the mortgage with it.
Edited, Feb 3rd 2013 7:10pm by Jophiel