Are you planning to run at medium settings?
I use intel i5 3570 with gtx 660 Ti SLi (thats two cards) and with that setup i run at highest quality and zero lagg at big zerg fights.. Usually at 60 fps all the time. This setup isnt expensive these days.
But it all depends on what quality you want to play with. I might also add that an SSD drive is allways good for gaming