New graphic card for XPS 15

The NVIDIA GPU is soldered onto the motherboard in the XPS 15 systems, so no you can't change it.  The Precision M3800 and then the 55x0 systems are the equivalents to each generation of the XPS 15 from the 9530 onward and are basically identical except they use Quadro GPUs rather than GeForce GPUs (and offer more component customization when ordered brand new).  But if you need something significantly more powerful, you might want to look at a Precision 75x0 system because the XPS 15s and equivalent Precision systems throttle performance significantly when they're under heavy GPU load since their target ultrabook size doesn't allow a sufficiently powerful cooling system to keep the system operating under sustained heavy loads at maximum performance.