ahmedhassan_92 :
The 840M will be the best choice as it is the most advance. It is also a really good GPU to play games at low to medium: http

/www.notebookcheck.net/NVIDIA-GeForce-840M.105681.0.html
Now we are getting somewhere. This means that gt740m is out of the question, but in case you forgot to read, the gt740m is of the 4gb version. Would that make it any more powerful than the 840m?
Having 4 GB of VRAM basically means the GPU can handle more textures and bigger textures (pictures to overlay on top of the 3D shapes it draws). A framebuffer for a 1080p render (at 16 million colors) is only 6 MB, so you need very little VRAM to actually render a scene. 3D models (lists of polygon corner coordinates) tend to be less than 1 MB because each additional triangle takes longer to render. So the vast majority of VRAM is used to pre-load and store texture maps. As long as all the textures the game wants to load fits into VRAM, having extra VRAM offers zero performance advantage.
If you're running games at medium settings, most of the texture maps will be a few MB at most (a 1920x1080 texture will be 6 MB, same as the framebuffer, maybe double that for MIP mapping - smaller versions of the same texture for objects which are far away). 4 GB is far in excess of what you'll ever need. Heck, so is 2 GB, and maybe even 1 GB. All that extra VRAM is only really useful if you're running with high or ultra settings with ultra-high textures. So no, I wouldn't consider having 4 GB of VRAM to be an advantage. I'd actually consider it a disadvantage as the laptop will use more power when gaming. It's just marketing people going nuts trying to pad the numbers on the spec sheet.
At this level of GPU, if you've got more than 1-2 GB of VRAM the only technical spec of the VRAM that should concern you is the memory speed. GDDR5 is the current best (fastest). But at this level of GPU I think most systems are going to be shipping with GDDR3. But if one of those laptops comes with GDDR5, that would give it a considerable performance advantage which might not show up in the benchmarks (which are probably from laptops which only had GDDR3).
Look up each of those video cards on notebookcheck. Better yet, see if they've reviewed the specific laptops you're interested in. It is hands down the best site for this sort of comparison, with comprehensive benchmarks, teardowns, and tests. In addition to the benchmarks, they also list the fps of various games on laptops they've tested which use those GPUs. So you can get a sense of the "real world" performance of those GPUs while gaming. I actually wish there was a site as good for desktop CPUs and GPUs.