Waste of money then really. May as well just get another 3870. I have seen that 4850 is on par with two 3870's 4870 surpasses it. Cant wait to see how the 4870X2 performs. just hope they get the price right, cus if I can bag it for about £300 I'm getting one!
I have herd that as well, but they have to have the same amount of ram, and all cards default to the slowest cards speed, so a dual 3870s may only perform as well as a single 4870 anyway, not sure if its worth it.