CPU: i5 3450
MOBO: Z75 chipset board
You can save some cash and get a i3 2100 and H61 chipset, the downside is you won't have much expansion capacity, but you will save money now and likely need to upgrade board and processor in 18months - 2years time when the i3 should be redundant.
RAM: You want 8GB for BF3 as to brands the cheapest 8GB set, RAM has very little performance hit on your setup, but look for DDR3 1600.
GPU: HD 6870/GTX 560ti
PSU: Corsair CX500
Screen: your choice.