The thing is those cards are slower then your 6850. The GTS450 and 250 have the same level of performance. I doubt either could do max(ish) details at 1050 or 1080 with PhysX on in games like Batman:AA. You'd really want a GTX460 or better for that. Keep saving.
i could get a gtx 560 ti -which would be faster than gts 450,gtx 460 1 GB and an HD 6850 - and i would be able to run physx on mafia 2 and batman arkham asylum - and as i liked arkham asylum so much -the upcoming batman arkham city.