I n order to do SLI the cards have to be the same , so you need two 470's or two 460's you can't mix the 460 and 470 in SLI. Physx takes such a little amopunt of video performance from a card that you can do it with almost any card down to a cerian model and you won't gain anything fro swaping the 280 out for the 460.
Since your friend is giving you this card it would be rude to turn around and sell it so the thing to do is keep it for physx and sell your 285. There is also the option to connect the two in SLI and see if by some luck they do work together , nothing would be damaged and it will either work or not work and since you are getting it for free you have nothing to lose by trying it.
Sell them all and get one of the new 600 cards coming out shortly, or (as you got one for free) give one to a friend.
SLI requires all the same cards, and to run 3 cards you need the mobo to support it as well (which is more rare than a board that does normal SLI). With mixed cards you can do the odd man out as physx like you are currently doing, but that is about all you can do with mixed cards.