PCPOP.com website has published a bunch of benchmarks where GT 240, low-end 40nm GPU from Nvidia, is used as dedicated PhysX card alongside with GTX 260. System used for tests.
Performance with PhysX content, running on dedicated GT 220 (marked yellow on graph) is compared to a single GTX 260, calculating graphics and PhysX effects simultaneously (marked green).
Batman: Arkham Asylum.
As you may see, adding GT 240 as dedicated PhysX processor boosts fps up to 30-40% in GPU PhysX games. Therefore, if you are looking for dedicated PhysX card (as part of traditional or Hybrid PhysX system), GT 240 might be the right choice.