Well, I have heard some reports about 'screen flicker' when people enable quartz extreme if they have PC Geforce 3's that have been flashed. However, lots of people have been using that card succesfully in some weird models, like the cube, so I'd guess that it'd work fine in apple's systems with ADC connctors. I'd try to get a confirmation from apple's marketing department before you decided to try an alternative means to your cooling problem.
However, your experience may be different than my own.