The specifications of new graphics cards will be similar to currently offered GeForce 920M, 930M and 940M. New chips are said to receive GDDR5 support, which would drastically improve performance compared to DDR3 memory they currently use.
Notebookcheck compare existing GeForce 940M DDR3 model to recently released GeForce 940M GDDR5 used in Microsoft Surface Book. Gaming performance increased more than 20%, just by updating DDR3 memory to newer standard. We should expect similar boost from new GeForce 900MX chips.
Source: VideoCardz and NotebookCheck