dx 11 = september, with it comes ATi and Intel Havok hardware acceleration. Now lets see, two massive companies, who will be soon incorporating gpu's into their cpu's so both will own the intergrated market completely, will have hardware acceleration aswell as Nvidia can use it aswell.
While Physx can be used by Nvidia only, the problem is, Havok games already MASSIVELY outnumber physx games, and the number of physx games that actually need hardware acceleration is what, under 10 and most of them do nothing useful.
So in a year, will developers be supporting an open format physics software that has hardware support for ANYONE that wants it, or a closed format Physx that only Nvidia can use. Developers will support what Intel does, its really that simple, with intergrated they already control like the majority of the graphics market, with discrete cards coming soon, they'll just gain more market share.
a 4870 is pretty much a steal at the moment at around £100, that would be where I put my money.
Keep in mind it supports dx10.1, which means as dx11 games come out(which will be fast as dev's have been ready to code for it for 2 years) dx10.1 cards will get boosts from several features, features Nvidia simply doesn't have. I wouldn't buy a non dx10.1 card so soon to getting dx11.