I need to replace the graphics card installed in my laptop (NVIDIA GeForce 8400M GT GPU)
Do i have to replace it with exactly the same one or can it be a different one? If it can be a different one, please advise which ones are compatible.
Bad news I'm afraid. On a laptop computer the graphics chip is part of the motherboard and to replace the graphics chip means fitting a new motherboard.
There is another alternative you may wish to consider. Have a look at this previous question: -