I'm pretty sure the new parts will be faster as they intend to use more transistors. The thing is they might not be faster in ways you'd expect, they could improve crossfire, tessellation and tweak little things.
But whatever happens I think AMD will be pushing limits rather than cutting costs as these GPU's have to hold out for until for at least 6 months and they shouldn't leave nvidia that much breathing room.
As price drops on the 5870, I imagine ATI be will switching production soon to their new cards. So 5870's will be dwindling stock, and in my opinion, it wouldn't be safe to make enough to justify a price drop. They can just slowly sell what they have sitting around as they prepare for a hard launch.
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