I can report that using a gtx 980 with an i5-4670k works extremely well, I have not come across any issues so far. Make sure you get SSD’s as well, they make a big difference.
A high end i7 would be nice of course, but plan for the future with upgradable hardware. Currently most of the high end i5/i7’s use the LGA 1150 socket, so you can always upgrade if you buy an i5 now, maybe an i7 1-2 years later when 4+ cores becomes widely used.
I highly recommend the GTX 970/980 though, it is an amazing card. 