You may face some problems if you leave the old drivers just like that.
First of all you need to TOTALLY remove all the old drivers. Use this link.http://www.wagnardmobile.com/DDU/
Select the respective company(i.e Nvidia or ATI) drivers to be uninstalled.
Use clean and shutdown option.(I think so) or use the option which says for installing new graphics card.
And then install the new drivers after rebooting.
It's better to totally clean up the drivers for better performance.