Swedish tech. enthusiast site SweClockers.com have posted speculation that the Green Team will have two new SKUs out in late July/early August based not on the GK104 of the GTX680, but rather a new GK106. According to views posted by an unnamed dealer the GK106 will feature 768 CUDA cores, which is exactly half that of the GK104.
Other technical specifications are even more uncertain; you have to anticipate that the GPU will need to be clocked pretty highly to compete against AMD's Pitcairn-based 7800-series with that number of cores. SweClockers note that a 256-bit memory interface for 2GB GDDR5 RAM have been spoken of in the same sentence as the GK106; this would certainly help any new GPU compete at >1080p resolutions with higher levels of AA.
A price of 2500 SEK/£225 inc. VAT for the 660 places it between the 7850 and 7870 and as the spiritual successor to the GTX 560Ti, but performance will be all important in this mainstream gamer segment. If the July release date proves to be accurate expect NVIDIA's GeForce Experience program to be a significant part of the marketing and features of buying a new GTX 660.
Source: SweClockers.com via TweakTown.com