Sorry (dont mean to be rude) but I had to laugh when I saw your append, about putting a desktop graphics card in a laptop.
A modern desktop graphics card is almost as big a some of the modern laptops, as seen in picture here:
http://www.msi-computer.nl/perscentrum/persber icht/persbericht_MSI_MXM_SLI-Card/images/IMG_9 475.jpg
To answer your question, I should imagine most laptops dont have a graphics card as such but a graphics chip soldered to the motherboard.
Changing this would be almost impossible.
Whereas parts for desktop PCs are standard and will fit inside almost any PC, components for laptops are usually specially made and so cannot just be bought off the shelf.
You may better to ask your question on a specialist forum about laptops or graphics cards. It would be a good idea to put the make and model number of your laptop as that is the first question anyone will ask.