I'd buy a GTX 260 for that price whether I needed it or not.
That's a smoking deal.
You could always just try it out. Play some Physx games, take advantage of the performance improvement, and then take the card out if you aren't using it any more. If you end up not needing it at all, sell it for a profit. That's what I did with my 9600GT. Its currently going into a system I'm building for a friend.