Situation 1)If you want to get the most bang for buck future proofing in Dx11, you should look at tri-cross-firing 5850s (new or used).--This setup alone will save you the money needed to get a bigger PSU (I think a 750W should be enough for tri crossfire), also will make your system run cooler (cross-firing the 58xx is more efficient in terms of noise and heat), and also give you great OC ability.
The 5850s are great for tri crossfiring, the 5870s are even better but the difference is minimal so you are better off with the 5850s.
PS- You'll need a decent pcie2.0 board that supports crossfire (16x/8x speeds on two slots, or three slots)....oh and a decent processor, preferably quad core
Situation 2)If you like to play some titles with PhysX and care about noticing it, at the same time would like a card that runs average but is in between a 5850 and a 5870, supports Dx11 and costs a little more, then go ahead with a GTX470-----However this card will hog power, and also require a bigger PSU ( I think..)
Situation 3)If all you play are Dx10 titles, then by all means stick to a Dx10 card that rocks fps on those titles, perhaps getting a used GTX295 isn't a bad idea as well (I think the GTX295 is Nvidia's best card)...or stick with a cheaper and similar performing radeon.