depending on what games you like to play stick with a 2gig minimum on video card memory.
Ram- 1600mhz or faster, low timings would also be nice i.e- 6-8-6-24 timings are pretty low. 6 gigs + . i think that quad channel ram is the new stuff.
CPU - i7 or i5
Power supply - 750w or greater -
Video Card - Nvidia 600 series-
Motherboard - whatever you want, I would personally get a sli ready board, so that in the long run you can sli whatever card youre going to buy, so that you can increase the life of the rig.
Harddrive - sata 6?
I wouldnt mess around with AMD cpu's , they are cheaper, but intel always dominates.
Chose a motherboard with good on board audio to save you from having to purchase a sound card.
L.E.D monitors 120hz are totally worth every penny.
Battlefield 3 will definitely require a 2 gig video card to play lagg free.
When you put it together, do not forget to do wire management so that your hardware stays nice a cool.
Let us know what you finally go with !