According to different sources, NVIDIA will launch its GTX 880 and GTX 870 in this fall 2014. The onboard GPU is GM204, which will be the successor of the GK104. We remember that the GTX 750 and 750Ti also equip with Maxwell Chip GM207, but there was no transition to 20nm, but this is also the evidence that even in 28nm there is still lots of space for improvements.
Nvidia GTX 880 and GTX 870 with Maxwell coming Fall 2014
Moreover, We are pretty sure that these GTX 800 rather come between October and November will also be engraved in 28nm, The semiconductor manufacturer TSMC is indeed starting production in 20nm, but it does not seem to apply to the kind of high-performance circuits used for both GPUs.
On this Fall the Maxwell chips will use their full potential, hence the manufacturing process of both cards is still 28nm. Both the parties AMD and NVIDIA will not use 20nm manufacturing process this year, It will be the worth waiting until 2015 as rumours indications.