Video cards can be upgraded, but it will just void your warranty on that certain part (per dell's policy). It really depends on what mobo/gfx combo you have as most are integrated. If the video card is integrated, than i can guarantee that your fucked. If its not integrated with the mobo than you "may" have some luck but your options will be limited.
Be fore warned, that laptop video cards are crazy expensive and may require additional mods to get them to run correctly.
If its a blue screen issue post your laptops specs along with the error your getting and what your doing before it happens. Could be a number of things.
edit: I upgraded my dell's 7900GS to a 7900GTX and it wasn't that hard but I did have to get a better PSU for it and flash the bios (which wasn't necessary)
and side note, firefox's spell checker sucks balls.