The problem is with Quartz Extreme. Even if you have an AGP QE compatible card, adding an additional PCI graphics card that does not support QE will disable both applications. As far as I know there are no PCI cards that support Quartz Extreme, and in iMovie's System Requirements there is no mention of graphic cards at all, let alone adding additional ones to your PCI slots.
It may be a small problem to some but those who have an additional card will find buying iLife '08 to be worth in reality 3/4 of the amount they've paid.

