First off make sure your computer has the latest NET Framework installed as the ATi control center uses it for fan speed controls and overclocking features.
Second, goto control panel and remove the nvidia drivers, make sure the card is removed from device manager. When it asks to restart, click no.
Third, there is a program called CCleaner and if you google you can download it for free. Run this program after you uninstall the drivers before you shut down and it will help clear out files as well as various other things that can cause driver/install problems.
Fourth, install the new ati card
Fifth, install the latest drivers off of ATi's site which is 9.2 I believe. And yes as someone already stated, the full package drivers are highly recommended.
Sixth, Open your catalyst control panel immediately goto the overdrive section and unlock it, you'll see your GPU temp on the right. If the temp is high such as around 70-80c then enable the manual fan speed controls and set it anywhere between 40-55%. The fan speed will also depend on how loud you want it, I believe mine is running 55% 24/7 and keeps the card around 43-50c idle/load.
Hope this helps!
