ATI cards work great on nVidia chipsets, but because you previously used an nVidia card you should get a program to clean out the drivers for that card and use it before installing the ATI video card drivers.
<font color=blue>Only a place as big as the internet could be home to a hero as big as Crashman!</font color=blue>
<font color=red>Only a place as big as the internet could be home to an ego as large as Crashman's!</font color=red>