If you have Foxconn BloodRage that means you have free 4 PCIe slots and the ability to go quad crossfire if needed.

I would say a 5870 CF since the future prospects are better, if DX11 is a sticker like the DX9 you can add cards till all your 4 PCIe slots are filled, in 5970's case you can just add another 5970 and that's it.

Also i would recommend waiting for the 2GB 5870's if priced not very high they will be a better choice.