I believe Sniper is correct in that you are not able to replace video cards in laptops. Though that may change sometime in the near future, I don't believe we are quite there yet.

Reportedly, Alienware is supposed to have a laptop with an upgradeable graphics slot which only begs the question...."Where do you go to find a laptop video card?"

From what I've researched, the only video cards I've been able to find for laptops so far are those that connect via a PCMCIA slot.