Apple states that you need an AGP-based graphics card with at least 16MB VRAM for Quartz Extreme (QE) to take effect. Apple lists the NVIDIA GeForce2 MX, GeForce3, Geforce4MX, or GeForce4 Ti, or any AGP-based ATI Radeon as compatible graphics cards.
What about users who have a PCI PowerMac, with a PCI graphics card which has 32MB VRAM? Apple does not enable QE by default, however, a third-party hack enables it. It is called PCI Extreme, made by a small company called Dangerous Wares. It is available for download here (http://www.versiontracker.com/moreinfo.fcgi?id=16006&db=VT).
I have a PowerMac G3 Rev A, with a Radeon Mac Edition (32MB DDR). PCI Extreme installed without any problems. I verified that QE was enabled with a utility called 'Quartz Extreme Check', available here (http://versiontracker.com/moreinfo.fcgi?id=15911&db=mac).
Apple chose not to enable QE on PCI machines because of potential performance problems. The PCI bus is much slower than the AGP bus, hence, throughput is much less. When doing operations involving the OS X window server, performance increases greatly. However, when doing CPU-intensive operations, performance may degrade on slower machines.