If you are in the position to easily sell your 4870, Id definately wait. Applying those profits against a new card should offset the differences in price, and youll have the perf of the newer card.
If youre into rumors, the most consistent and best thought out one is having the 5870 as equal to somewheres in between the 4870x2 and the 295, so, if you wait, then youll know, and, also get a slight upgrade with also Having a card which is DX11 capable