You're talking about the high end laptops. Custom jobs. BIG BUCKS. Not just any ole laptop will let you swap out the graphics CHIP. They are hard soldered to the motherboard and are wired up to "borrow" a portion of the system memory to function.
About the only thing you might be able to do is install the max memory for your system and see if the video subsystem will let you up the amount it "borrows". That setting is in the bios, by the way.