3DCenter has detailed the specifications of NVIDIA's Upcoming 28nm Kepler based GK106 "GeForce GTX 660" GPU which would fall under the Mid-Range Segment of GPU's.

~ 210mm ² chip area under the 28nm fabrication by TSMCDirectX 11.1, Kepler architecture2 Raster Engines (GPC)4 shader clusters (SMX)768 (1D) shader units64 texture units (TMUs)24 raster operation units (ROPs)192-bit DDR memory interfaceMemory Buffer: 1.5 or 2 GB GDDR5PCI Express 3.0 (backwards compatible with PCI Express 1.x and 2.0)Games ~ 130 Watts power consumptionProduct name: GeForce GTX 660Launch: Q3 2012

The site details that the GK106 Chip is made up of768 Shader Units (Half the amount of GTX680 Cores), 64 TMU's and 24 ROPs split in to 4 SMX Clusters. It would be based on the 28nm Kepler Architecture with the die size ranging around210mm ². The GPU would hold a 192-bit GDDR5 Interface and the card would consist of a 1.5GB or 2GB Memory buffer.

The GPU would feature a 130W TDP design and would be powered through a single 6-Pin Connector. As far as performance is concerned, The GPU which would belabelledas the GeForce GTX 660 would have to be clocked at around1006 MHz chip clock to provide 50% of the computing performance of GeForce GTX 680 and a very high1359 MHz clock to retain 67% performance of the GTX 680 which is very unlikely. The AMD Radeon HD7870 has around 67% computing performance of the Radeon HD7970 at its stock clocks so its pretty clear that NVIDIA won't be position it against the Pitcairn Parts.

A more reasonable competitor seems to be the Cape Verde XT and Pitcairn Pro HD7770/HD7850, The card replaces the GTX 560Ti/560Ti 448 Cores and would be priced similar to those around $220 Price Range. Its expected date of arrival is in late Q3 2012.

NVIDIA has two GK104 parts incoming in May 2012 to compete against the Pitcairn XT and Tahiti Pro GPU's. Details on those below:

NVIDIA to Launch GeForce GTX 670Ti and GTX 670 in May 2012, No GTX 560Ti Replacement till 2H 2012