Should be fine for ultra high settings in both of those games, BF3 might not be completely smooth though. I'd forget Crossfire personally, It's not really necessary on a single monitor. I would get a better single card so you can avoid unnecessary heat/compatibility/noise issues.
You could get a cheaper board like the PRO3, a cheaper PSU like the CX-430 or CX-500 and upgrade to the HD 7850 2GB. That gives performance similar to the HD 6970 with a lot less power consumption than the HD 6870.