Well, "the faster the better", but really it depends on many factors, like amount of money you want to spend, performance you want to get, noise, power consumtion, etc.
High end GPUs like GTX 570/580, GTX 660Ti/670/680 should be enough to play with maximum PhysX effects on a single card, in any current GPU PhysX game.
As for dedicated PhysX GPU, most common choise would be something like GT240/GTS450/GTX650.
However, there are several recomendations:
1) Don't fall for buying cheapest dedicated GPU possible (something like GT9400 for GTX680) - it will only bottleneck your system, resulting in lower fps than using single card
2) It is also unreasonable to buy high-end GPU for PhysX only (something like GTX670 to pair a GTX680). In this case, two GTX670 in SLI will fit better.
3) While choosing a dedicated card, look at amount of CUDA cores and GPU clock. Memory amount is not that improtant (512 will be enough).
And here some benchmark links:http://physxinfo.com/news/tag/benchmark/http://www.overclockers.ru/lab/41183_3/NVIDIA_PhysX_ot_A_do_Ya.html
(russian article, but with huge amount of cards tested)