i5 4670, no point of getting the k because you can't overclock with your motherboard.
But what's the difference between the 4670 and 4570? Which one should I get?
I would spend the extra and get a Xeon 1230 v3 to give you a bit of extra life out of your system as new games going forward will have be more like Crysis 3 and BF4 in terms of multithreaded support.
My friend has a Xeon, and its only advantage in gaming is the larger of threads, but if it was only by that I would be buying an AMD FX-8350.