The older GTX 660m is a little more powerful than the 940mx, but definitely less powerful than a 945m... at least according to how Notebookcheck.net ranks GPUs based on game benchmarks. Click the link below to see some benchmarks for the GTX 660m, just be aware that the most recent games benchmarked are from 2014. Do a search for "940mx" and you will see in the middle column that both GPUs are ranked near the bottom of Class 2 GPUs.
https://www.notebookcheck.net/NVIDIA-GeForce-GTX-660M.71859.0.html
If you can wait a bit before buying a laptop, then I recommend you wait. Nvidia released the GeForce MX150 which is basically the successor to the 940mx; that and the MX130 are nVidia's new "mainsteam" GPUs. The MX150 is close to the...