I would go with the 660 Ti and save some money, I have seen much reviews and I think the difference 660 Ti and the 670 is not very substantial, except in higher settings. Max about 20% slower. But, the 660 Ti can be bought $100 less.
EDIT: Just found a 7970 for $420, with a 12% discount? maybe you should also take a loot at this? It trade blows with 680 so it could be a good option too.http://www.newegg.com/Product/Product.aspx?Item=N82E16814129264