Drop that PSU quick, and go with what recon-uk suggests. 80 Plus Bronze. Check. Single Rail. Check. Modular PSUs are a very nice feature. I have a Seasonic X650, pretty sure some of Corsair's products are Seasonics inside.
I'd try overclocking the FX 4100 first to see if BF3 shapes up. Then try inching up the 6850s. If all else fails, an Intel i3 2100 or an Intel i5 2500K and a new (at least PCI-e x16,x8) LGA 1155 motherboard with 2 slot spacing in between the 6850s is about all I can recommend in all confidence. Wait for a week or so though, and let Ivy Bridge shake up the market. Might mean better prices across the board.
I'm pretty sure an FX 8120 would not perform much better. A Phenom II X4 or X6 might.
As to your BF3 performance. Is the problem a stuttering of sorts or just that on higher settings the game is unplayable? I've seen a single 6850 with an I3 2100 perform higher than medium.
And as an aside: is this a pre-built PC? If so how long ago did you get it? The thought of returning it, and building something better in the same budget with your extra 6850 could be possible. Its just that AMD's newest FX processors are really not great for gaming (on the mid-high end that is), and that Hantol PSU is a time-bomb.