For $700 you are not likely to find something that will play Fallout 4 at 40 FPS and 1080p unless you want to play at low graphic settings. I am not familiar with XCom. For $700 you can only get a laptop with a nVidia 940m. Fallout 4 will use an updated Creation Engine that Skyrim also used. That means Fallout 4 will likely have higher system requirements than Skyrim.
The following link shows benchmarks for the 940m, Skyrim is the last game in the list. With high graphic settings at 1366x768 resolution the average FPS amongst the 3 laptops tested is 34 FPS. At 1080p and ultra settings the average FPS is 17.
http
/www.notebookcheck.net/NVIDIA-GeForce-940M.138027.0.html
Based on those results if you are looking to play Fallout 4 at...