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 performance of the GTX 960m based on a few benchmarks in the article linked below. Be aware that actual benchmarks can vary depending on if the laptop has a dual core / quad core CPU and the actual clockspeed of the different Intel CPUs, and if the game can take advantage of more than 2 cores.
http://www.ultrabookreview.com/16104-laptops-nvidia-mx150/
Below is a video review of an Acer Aspire laptop with the MX150 GPU. Supposedly it will cost $600; a link is provided to Amazon in the description, but the link does not actually bring you to the laptop. I am guessing it is not actually available for sale yet. Overall, the laptop seems pretty decent with a Core i5-7200u and an okay IPS 1080p screen, but it does not have a backlit keyboard which is the only thing I find lacking, then again it is a relatively inexpensive laptop.
https://www.youtube.com/watch?v=al_5-BDDFPs