No need to reformat... yet.
If you're going from nvidia to nvidia, no problem. Or ati to ati, no problem.
If you're going from one to the other, you have to remove all traces of the original drivers BEFORE you put the other card in. Nvidia's drivers are pretty coopperative (I'm told) and clean up after themselves. ATI's aren't, so you need to get their uninstaller (it is a stand-alone uninstaller). No matter which you do, be sure to browse through your registry to make sure that nothing is left there, as well as browsing your harddrive to make sure that nothing was left there either.
Reformatting should always be a last option.