The GTS 450 is so new there are no real reviews out for it yet but from what we know and the leaked testing that has been about the 5770 seems to have the egde over it so that card would be the answer to both questions 5770 all the way.
Can the 450 be overclocked to compensate ? Dont know yet
Can the 450 run PhysX at a decent level ? Again this we dont know but from what i do know i doubt it.
If you have to buy now then get a 5770, if you can wait a couple of days for some reviews of the 450, just to check then do so.