Battlefield 3's performance isn't very CPU-bound, but GPU-bound. The 4-cores is good to have though, you should be fine CPU wise.
The GT640 is a tad bit stronger than the Radeon HD 6670, no? When I first got BF3 I had a 6670 at 1600x900 resolution.
It was very playable on the lowest possible settings, even with FXAA on low. (As compared to the off option.)
And by 'playable' I mean approximately 35-50 fps on average; When things got busy(64-person server) I then saw dips into the high 20s.