A report says the cards will be based on the 28nm process, and are due out in the last quarter of 2014.

Nvidia Coporation  New details on Nvidia’s Maxwell based GTX 870 and GTX 880 leak

Nvidia will launch its next generation of graphics cards in the fall, and they will be based on the Maxwell architecture produced on the 28 nanometer process node according to a report by Sweclockers.

Nvidia’s GTX 870 and GTX 880 will be launched in the fall — likely between October and November — and will use Nvidia’s Maxwell architecture on the 28nm process node. As reported earlier, Taiwan Semiconductor Manufacturing Company (TSMC) is having difficulty gearing up for a shift to the 20nm process node and firms like Nvidia and AMD would rather get chips out with the larger process instead of waiting on TSMC.

The GPU powering the GTX 870 and 880 will likely be the GM 204, the successor to this generation’s GK 104. However, this is not yet confirmed and it may be the case that the GM 204 won’t be officially used until Nvidia can get access to a production line that can fabricate at 20nm.

Source: Sweclockers