The problem is that the graphics chipset is part of the notebook's mainboard, so physically it can not be exchanged. Even if the graphics card is on a daughterboard, the connections are typically proprietary unlike standard connections found on desktop hardware.
Furthermore, no port that the unit offers be it Hi-Speed USB or any other has enough bandwith to allow a high performance graphic chipset to be connected adequately to the system. That is why external graphic cards do not exist.
Today's notebooks including yours do have does have the high speed and high bandwith AGP or PCI Express for Graphics connections but they are used by the internal graphics chipset.
