Compressor 1.x is not compatible with the Power Mac G5 (Late 2005) or iMac G5 (iSight).
Compressor 1.x requires a computer with an AGP video card. PowerMac G5 (Late 2005) models (released October 19, 2005) and iMac G5 (iSight) models feature newer PCI Express video cards. Because of this, Compressor 1.x is not compatible with these computers.
To use Compressor on Power Mac G5 (Late 2005) or iMac G5 (iSight) computers, please upgrade to Compressor 2. Compressor 2 is included with Final Cut Studio.
Compressor 1.x was included with Final Cut Pro 4, DVD Studio Pro 2 and 3, and Motion 1.
Find more information about Final Cut Studio:
http://www.apple.com/finalcutstudio/
302667: "Final Cut Pro 4.5 HD and earlier not supported on Power Mac G5 (Late 2005) or iMac G5 (iSight)"