The first question is, what PhysX games do you play? Sometimes people get confused and don't realize that the list is vey specific: http://physxinfo.com/index.php?p=gam&f=gpu
Then if you are playing one of those games, the dedicated PhysX card needs to be a good match for your primary graphics card. With a good pairing, a dedicated PhysX card can improve your framerates roughly by 25% as a rule of thumb. If you are not playing one of those games from the list then there will be no performance difference.