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from Wikipedia:

The Power Mac G4 was a series of personal computers that was designed, manufactured, and sold by Apple between 1999 and 2004.

They used the PowerPC G4 (PPC74xx) series of microprocessors. They were heralded by Apple to be the first personal supercomputers, reaching speeds of 4 to 20 Gigaflops. They were both the last Macintoshes to boot natively into Classic Mac OS and the first to boot exclusively into Mac OS X.

The original Apple Power Mac G4, code name “Yikes!”, was introduced at the Seybold conference in San Francisco on August 31, 1999, with 400 MHz, 450 MHz and 500 MHz configurations available. In October 1999, Apple was forced to postpone the 500 MHz because of poor yield of the 500 MHz chips. In response, Apple “speed dumped” the processor speed in each configuration by 50 MHz but caused some controversy by not decreasing the price of the machines.

Another generation of Apple Power Mac G4s were introduced on August 13, 2002, featuring both a new Xserve-derived DDR motherboard architecture and a new case design, known as “Mirrored Drive Doors” (MDD). All models were available in dual processor configurations running at 867 MHz, 1 GHz or 1.25 GHz. As with the Xserves, the PowerPC 7455 CPU used does not have a DDR frontside bus, meaning the CPU could only use at most 50% of the new system’s theoretical memory bandwidth, providing no improvement over previous models. The rest was available to the graphics card and I/O systems. A dual processor 1.25 GHz model would be the last Power Mac G4 the company offered to the public after the announcement of the new Power Mac G5, introduced in June 2003.

source: wikipedia