You're really asking this question on a bitcoin forum? Everyone in here's gonna give you the same answer. You're just begging for someone to tell you to buy a 7970. Do iiittt....
I almost didn't start this thread but decided to anyway just for the discussion, and that maybe just maybe someone will say that magic something that decides it for me. Wishful thinking... probably.
600MH/s is about ~$1.50 a day (after you minus electricity). That's something the 670 wont give you. To me, that alone is worth the "cost" of not going with Nvidia (Nvidia drivers are the shit).