Bub, are you sure you removed any trace of the ATI drivers?
I have seen a lot of people having problems with conflicting reminants of the ATI drivers when installing a NVIDIA card. Altough this is mostly on the gamer sites, there have been mentions of game-slowdown, gliches and artifacts. Even sound distortion seem to occur in some cases (don't ask me how that's possible, but I think I saw this mentioned on the X3 forum - that is ... the game, not musical sofware

). If it happens in games, it can also happen in other software that is depending on graphical parts of the interface..
I do not say this is the case in your system, but it may be worth to take a look at it. As I seem to remember there is a application that removes all traces of the ATI driver. I cannot remember the name at this moment - sorry...