You could always get a dedicated Physx card and use the hacked Physx driver to run it. I've got a flashed HD6950 and I am running a GTS250 to run my Physx and it works great. I haven't bought BL2 yet to try it, but all of the other games that I have tried with Physx seem to run great.
I'll have to look around for the guide that I used, but I can tell you how it works if you really want it.
Now the question regarding selling your card being worth it, that's your call. If you have the cash then I guess. personally, I like my 6950, which is why I set up a dedicated Physx card.
Now you can't hack Physx ONTO the ATI card, since the card just does not have the hardware for Physx, but you can add the Nvidia card in there and run Physx on that. There are hacked Nvidia Physx drivers so you can run the Nvidia card with the ATI card.
eh. it's neat the first hour or so, but after that, you're far too concern with the actions and progressing through the game that you don't notice it much anymore. if you're willing to spend, i guess it's ok. if you're not thinking of using a hybrid AMD + Nvidia GPU + hacked drivers build, and was going for say a single 560ti, you'll experience some performance hit.