Well, the 460/Hd 6850 will be better for bf3. ( still, no maxed out settings, but with the cpu that's not possible anyway, there might be just this litlle bit of a bottleneck allready with the 460/6850 )
at that resolution and with a dual core i wouldn't go any higher than a 5770/6770. both should allow you to play BF3 fairly well at such a low resolution, but the processor will bottleneck that game quite a bit. its very multicore focussed.