youll get a small boost
bf3 is gpu limited as shown many times
when it first came out it showed a phenom even at stock would be identical near enough to a i2500k at 5 ghz.
not saying dont upgrade but a lot will say yeah big leap or whatever when basically youll get a few fps more maybe
you may get that much by slighltly overclocking your card or close.
depends on the money really .
Those benchmarks were for singleplayer. On 64 player multiplayer maps Sandybridge CPU's blow anything AMD has away.