Waragainstsleep is right.  You can buy G4 500-1ghz or G3 1GHz upgrades for your G3.  Keep in mind though that these CPU's are re-engineered to work in beige and blue G3 towers.  A G5 will never run in that system.

The only factory G4 chips that will work are the ones in the PCI graphics G4 tower from 99.  After these models Apple totally changed the CPU socket.
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Mac mini C2D 1.83GHz - 2GB RAM - 200GB 7200rpm HD
Sawtooth w/G4 1GHz - 1GB RAM - SIIG SATA w/2x1TB HD
B&W w/G4 600MHz - 1GB RAM - 2x80GB HD
Sawtooth G4 400MHz - 512MB RAM - 40GB HD
Dell Ultrasharp 2007WFP 20" Widescreen LCD
2.94TB Total Storage