The GTX 970 is below 350$ and it will be a huge improvement over the 660.
The only question is which one of the range should you get, this one looks very good. Avoid ones that people review to have coil whine (high pitched noise from the fan)
http://www.amazon.com/EVGA-GeForce-Superclocked-Graphics-04G-P4-2974-KR/dp/B00NVODXR4/ref=sr_1_1?s=pc&ie=UTF8&qid=1416770809&sr=1-1&keywords=gtx+970