How to add Graphic Card to Virtualbox using Windows XP

Nope. First, the cost to have the GPU properly soldered to your motherboard would be prohibitive. It pretty much has to be done when the other components are attached. Second, your laptop doesn't have the cooling system needed to cool the GPU. Laptops bought with a discrete GPU included the necessary cooling.

If you want a laptop with discrete graphics, you need to buy one. About all you can comfortably change on a laptop is the RAM and the hard drive or SSD.

It dosen't emulate either one of them - its a 'unique' graphics card virtualbox supports with its own drivers (wikipedia has information on what devices that virtualbox virtualises). This thread suggest starting with the nvidia version, installing virtualbox guest tools, with some settings tweaks (enabling openGL), setting up the virtualbox guest drivers and so on.