Apple has released updated drivers for its nVidia graphics cards. The new version, 3.0, includes better support for operating with the Matrox RTMac graphics card, and other improvements. From Appleis update notes:
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.
Download the nVidia Driver, version 3.0, at Appleis Knowledge Base site.