The video GPU is integrated into the motherboard. Here's a list of your possible replacements: http://www.ifixit.com/Mac-Parts/PowerBook-G4-Aluminum-15%22-Model-A1095#0-Logic+Boards
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Here's the replacement instructions: http://www.ifixit.com/Guide/Repair/Installing-PowerBook-G4-Aluminum-15-Inch-1-1-5-GHz-Logic-Board/231/1
The video GPU is integrated into the motherboard. Here's a list of your possible replacements: http://www.ifixit.com/Mac-Parts/PowerBook-G4-Aluminum-15%22-Model-A1095#0-Logic+Boards