Apple has updated the drivers for the NVIDIA graphics card to version 2.6. The update includes better support for working with Matrox RTMac video cards, and other enhancements. From Apple:
The NVIDIA Driver Update improves the stability and compatibility of NVIDIA cards in the PowerMac G4 when used in combination with the Matrox RTMac Card.
This update insures better quality playback and filtering.
You can downloand the update from the AppleCare.