maybe you can wait a bit until the game comes out before spending your money on dedicated physx card. the thing is there is a chance your can't enabled the advance physx option because borderland 2 might need latest version of physx to enabled the advance physx option.
AFAIK to enabled physx with AMD as primary gpu you need to use hack. the problem is the way i heard it the hack was using older version of physx and i heard there is no new hack done to more recent version of physx.
so my advice is you should wait a bit. only go buy the card if you can confirm there is a way to make the advance physx option working with the latest bersion of physx (with amd gpu as primary).