help with choosing a graphics card for a LAPTOP

thought the options for changing the graphics card on the ASUS.
options -
intel i7 4 core 3.4 GHz
8GB RAM
motherboard asus x750jn
nvidia gt840m 2 gig
vin10, 64
the cooling plant is clear

want diskretnoi graphics card on 4гига. be all this hardware update? and which graphics card will stand up on that Board