BF3 is going to be very CPU demanding but it's also GPU demanding.It's a really powerful game engine.Although your q8300 will be bogged down from the weight of BF3.I suggest upgrading if you can.Or O.C. if possible.
The prices are with psu I will buy both corsair builders 430/600 respectively will be playing at 1600x900 possibly 1080p
Also the 6870 doesn't come with shogun so that's a bit of a choice ender almost
I am defiantly buying the card tomorrow so if someone wants to give there opinion.
If you can muscle it I would suggest a 6950 or a 560ti. In regards to the difference between a 6850 and a 6870, it's nominal at best. I would just get the 6850 if you want your shogun your net fps change would be 1-2 at best (fabricated numbers, albeit darn close I bet!)