that build with the 7850 is still good, since you arent playing at 1080 you would actually get a performance boost. you would be hitting the 60 fps mark, constant 45-50+s for sure. it would be surprising if you didnt get a constant 60 fps even
heres a comparison vid of a 7850 on ultra @ 1080 vs a 660 ti
30+ FPS is perfectly fine for eyecandy. you're playing at the same FPS as current gen consoles, but with more eyecandy. only thing is, once you know how 60+ FPS looks like, everything seems choppy.
and lemme post another build that would blow everything out the water, especially for a gaming buildhttp://pcpartpicker.com/p/w9sK
its not a K processor so you can't overclock it BUT, this completely eliminates the chance of bottlenecking your system, which you would definitely run into if you chose the i3 for being a dual core system.
to answer the i3 question:
older/recent games still only use up to 3 cores and a majority still only use 2, but newer titles are starting to utilize these cores. BF3 uses the Dice Frost Bite engine, which actually uses 4 cores (especially for 64 player maps). the CryEngine 3 has been reported to use 6 cores. so if you want to keep your rig up to par with the newest games AND play online, I suggest anything BUT a dual core now.