You cannot add a GPU - the nVidia version uses a different system board with the nVidia chip soldered to it.  While you can replace the mainboard, doing so will (1) cost you your warranty, and (2) be exceedingly expensive, since you will be replacing both the mainboard AND the processor -- the processor is soldered to the board).

If you need the faster video and the system is less than 21 days old, call Dell and arrange a return and purchase the upgraded model.    There's no practical way to upgrade what you have.