NFS The Run does not support GPU Accelerated PhysX. I think it does use the PhysX engine, but all calculations are done on the CPU. A list of games that do support GPU Accelerated PhysX can be found here
The 7750 should absolutely destroy a GT 430 in terms of performance. I don't see how the Nvidia card would make the game look better unless the game has some graphics features aside from PhysX that have been restricted to Nvidia cards, or you simply weren't running the game at the same settings when comparing the two cards. It could also be that texture filtering or other options weren't configured properly with the Catalyst Control Center, or some other configuration issue.