2x HD 3870 will be faster in most situations, but there are sometimes that Crossfire/SLI don't work well, in those situations the GTX will be faster.
I'd consider waiting for the HD 3870 X2, a single card that will have two RV670 chips onboard. It'll probably end up cheaper than 2x HD 3870s and *may* have better performance. It's not as concrete, but there are also rumors that nVidia will introduce a dual-G92 card. Regardless, we know that the X2 is coming sometime later this month, along with the rest of AMD's 55nm GPU lineup.