Quartz Extreme doesn't work with PCI cards by default. Some people claim that QE improves performance when enabled with a compatible card, others claim overall system degradation or destabilization.

Anyway, Apple doesn't support QE with PCI cards even if the card's chipset supports QE. You can enable it yourself by tweaking some text files, or you can use the handy dandy PCI Extreme tool. Note, however, that many people report serious problems after enabling QE for PCI cards on OS X 10.4.3+, so use caution if you are on Tiger.

Reports of success (or lack thereof) with Quartz Extreme enabled for PCI cards
DownloadPCI Extreme

Peace,
Drew
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Power to the PowerPC!