The HD 4870 is not a competitor for the GTX 280, that's the HD 4870 X2's job. The 4870 is aimed at the huge gap between the 9800GTX and the GX2.
From what we know already, the X2 wins by almost 15% over the 280 in 3DMark Vantage with pre-release drivers and the price tag should be around $475-$550, thanks to the strategy AMD has adopted for the high-end GPU market. They believe that the days of monolithic dies and huge power requirements are over, and that multi-GPU is the future. RV770 is reportedly only 230mm2, which means yields will be high as there is less chance of a defect occuring and therefore means it is cheaper to produce. The GTX 280, because of it's monolithic 575mm2 die size and low yields, is around $650, but of course, you have to remember that Vantage doesn't mean everything. We'll have to see who has the right idea when it comes to real benchmarks.
One thing I'd love to see is the X2 using a shared memory controller.