This guy used a 295 (of which only one GPU is used, so a GTX 275) as PhysX for a 480 SLI setup.
I'd say a 470 would do you very well in this scenario, though there's only an average 1FPS increase in the above chart, it should increase for the 580 alone, let alone SLI. (plus not accounting max rate)
But a 460 would add alot less thermal output inside your case.
Why not wait for the release of the 560 and get that? If heat output is anything like the 460, a good MSI Cyclone or Asus DirectCU will be a brilliant PhysX, and as powerful as the 470.
If you can't wait for the 560 to come out and prices to drop, the 460 is cheap and 336 Shaders versus the 285's 240 Shaders will also give it a good lead in PhysX rendering capability AFAIK.
EDIT: Sorry, if it's just
between the 470 and the 285, the extra VRAM on the 285 won't matter as much as the Shaders/Cuda Cores or GPU processing power.
The 470 will be the superior PhysX card, but it'll heat up the 580 above it, aswell as add to case thermals as I said. Edited by EvoBeardy - 12/23/10 at 6:01pm