If you already have an HD 4870 X2 then you have no real benefit to spending that much money any any card for a while.
If money is no big deal to you then get the HD 5870 and whatever better card comes along later. But if money is an issue, don't bother for a long while.
I would wait until the 5870x2 comes out. If you're dissatisfied with the frame rates yo uare currently getting with your 4870x2, then you can sell it and get the 5970. With that, you should expect to see excellent frames in the most demanding games.