Those are both very similarly performing cards but they will not let you play many newer games on high settings with good FPS. I have a considerably more powerful HD7970M in my laptop and it runs just about everything on High or better but it also costs a LOT more and it's still extremely weak compared to its desktop counterpart.
http://www.notebookcheck.net/NVIDIA-GeForce-GTX-850M.107795.0.html
http://www.notebookcheck.net/NVIDIA-GeForce-GTX-765M.92907.0.html
Look at all the yellows and reds in those lists under "Game Benchmarks" and you will get an idea of what those cards are capable of. As you might notice, almost all the games that got a reasonable FPS on High were tested with a resolution of 1366x768 so expect to have to turn the resolution down in favor of FPS.