Laptop graphics cards are much much much smaller than desktop cards. It is quite rare to find a decent laptop with removeable graphics cards. Gateway made one, but nowhere I looked sold graphics cards that you could put in it. Laptop graphics cards are much less powerful than their desktop counterparts. My 9600GS laptop graphics card gets an Aquamark score of 68000, where as my desktop version gets a score of 88000. That's quite a big difference.
As a matter of interest, what do you think laptops would look like if they had, say a 9800GX2 or a 4870X2 in them? Those cards are dual slot, nearly 20cms long, and produce enough heat that they rival your microwave oven. Might be something to try out in the future if it's ever possible.