TopLuca :
If you really want the best info you should send a PM to our Nvidia expert 17seconds , This guy knows everything about Nvidia .
Imo , It'll depend on how much you're going to sell the 750ti , the heat is negligible for me as the 750ti is cool and doesn't draw much power , either ways message the guy as he's very good at Nvidia's products.
I do agree with huilun that not a lot of games use PhysX however you can still use the card as a backup or a spare card for a next build , its better than selling it and losing about 40% of its original cost.
Thanks for that. I'll save him the trouble. Basically, it doesn't seem like you have to decide right now. I would try out the GTX 750 Ti as a dedicated PhysX card, and if you don't like it, or it doesn't do what you need it to do, then sell it.
http://alienbabeltech.com/main/using-maxwells-gtx-750-ti-dedicated-physx-card/
I can tell you that I finally got around to playing Borderlands 2 on my GTX 780 Ti, and I'm seriously considering installing my old GTX 580 for PhysX just for that game. There are just too many places during heated battles where the PhysX effects cause noticeable choppiness. The GTX 750 Ti will do a great job while using little power and generating less heat. It's probably the ultimate PhysX card.
First make sure that you are playing PhysX games, by checking this list.
http://www.physxinfo.com/