since you already have a 460, get a second one while there is still stock available. Otherwise you will have to find a good second hand one. a gtx560 is essentially an OC'd 460. With 2 x 460's you wont need a dedicated physics card, fps will be good. "Dont go in SLI because it cause a lot of problem... Make the GTx 460 PhysX and make this Evga 460 Primary and is also just $40 more but is worthed... http://www.newegg.com/Product/Prod [...] -_-Product" that is horrible advice.
anything above a gtx 260 would be waste of for physx. Definitely go for the sli as there isn't a lot of games with physx.With SLI you'll notice better performance all around as a mater of fact SLI will out perform single card with physx on physx enabled games.