Well anything below 30fps is unfavorable, but the gtx670 will do fine. My question is do you plan on doing any overclocking to your GPU once you get it. Because as I said earlier, an overclocked gtx670 can match or slightly outperform a gtx680. Now that you mention a hd7970, if you do overclock that then you will get some great performance. For the price, I would go with a non reference gtx670, or if you want to spend a bit extra a hd7970. Some people say that the fan on reference hd7970's can get pretty loud though. I would not really take the gtx680, don't get me wrong, its a great card, but an overclocked gtx670 can do the same. However a hd7970 with a custom cooler costs about ($399) which removes any fan related issues. If you overclock the hd7970 it will outperform the gtx670 in certain cases.
Here is one from gigabyte:
http://www.newegg.com/Product/Product.aspx?Item=N82E16814125413
Here is one from sapphire:
http://www.newegg.com/Product/Product.aspx?Item=N82E16814202008