HEXiT :
bf3 is pretty much a tri/quad engine in other words it really does benefit from up to 2 extra cores. even with a 4890 you should be getting more than 40 fps on medium as your not using the dx11 feature set which is what hammers performance in this game.
but the reason your not is because the cpu is holding the card back. i guarentee that if you run gpu-z you will see your gfx will struggle to get more than 50% usage. and you will get terrible fps bounce (fps goes from good 30+ to bad 5+fps)
so i would recommend you get a x4 athlon as soon as your budget allows, if you can. that way you wont have to replace everything.
Right, I made a Tomsharware account just to raise a point on this old thread for anyone thinking of getting an x2 270.
Before my 6850 blew out due to my cat knocking a coffee into the hdmi slot i could run bf3 on high and some max settings with no fsaa, using a 270 clocked at 3.876ghz with a stock 6850 and 4gb ram.
With good fps, on single player only heavy partical scenes the fps would drop, online was nearly faultless in terms of fps stability. I agree that a 965be which is about £20 or 37$ more expensive is better, its a quad core and both can be clocked to 3.6 or 3.8 on stock cooler stable.