I don't think you can change from an AMD GPU to a NVIDIA GPU in a laptop. The average price for a new 2gb 680m is around $600.00 I've seen them cheaper used. I haven't been able to find what a 4gb version costs. Its generally not worth it to upgrade a laptop gpu.
Gamefaqs game rating system : 10 = Best Game Ever. 8-9. Crushing dissapointment. Below 8 :Total Garbage. This is getting ridiculous. people agreeing so far 112