replacing a graphics card on a laptop

is possible to replace a ati radeon 7500 with lets say a better graphics card? the ati radeon for my inspiron 5100 blows and i want an upgrade. it just can not handle the latest games and it has problems with direct x9. many objects turn solid purple when i install it to play a game, so i do not have it installed anymore. are the new ati radeons smaller than the ati 7500? cuz if so, maybe  i could pack it into my laptop.  maybe it is just the way i customized my laptop, but my laptop seems to be heavier than ther laptops, so maybe my older graphics card is bigger and heavier... any comments?