Yep, a 3870x2 would work fine since the Crossfire is basically built into the card itself.
However, why are you looking at nVidia motherboards??? I would only ever recommend one if someone really wanted to SLI two nVidia graphics cards... If they were just going to run one card, be it nVidia or ATI, I'd say go with Intel for better stability, support, performance, etc (in most cases anyway). Maybe if you already had the nVidia chipset I could understand putting in an ATI card, but if you're building from scratch get an Intel board with your ATI card. Get an X38 if you want to leave the door open for adding a second ATI card for Crossfire some day, or get a solid P35 board (GA-EP35-DS3L for like ~$100) if you just plan on using one graphics card.