unpluggin the old card and pluging a new one back in is the easy part.
if your switching from "ATI" to "nvidia" or vise-versa though... sorting out the posible driver conflicts in windows could be more chalenging if it were to occure.
*note, more newbie info* - if your old card was PCI, and your new one is AGP... The new card will not go into the old slot. it will instead go into the AGP slot if your bourd is equiped with one. All newer bourds are, but some older ones are not.
you will still have to remove your old GFX card though even if thy dont use the same slot, otherwise thy could conflict.
*super newbie info* - o yea, PCI-E graphics cards do not go into normal PCI slots. you would need a new bourd with PCI-E to use it.