You should always clean out your old drivers before installing in new ones, doing so will give you better stabily and less chances of your system crashing, also a notice gain in speed.
For those that want to easily do a clean driver install of their ATI drivers or nvidia driver in their system you can download "driver cleaner"
It's a small ,simple program that do just that.
you can get it here
http://www.driverheaven.net/cleaner/ Since you're there, check out their forum , where you should be able to find lots of info on what ,how, not, do, etc on video drivers/cards.
http://www.driverheaven.net
Warning, if you're using Driver cleaner don't place the New drivers that you're going to use in your ATI folder cause it's gonna get delete with the rest of the old ATI stuff. After find out that my whole ATI folder is gone, and my new drivers are no where to be found in my system, I went online to ATI's website to redownload it, but for some reason their driver page wass DOWN at the time,.. Talk about unlucky

Anways, just make sure yout new drivers or any important files is somewhere else.
