"IBM has made public a Fall Processor Forum 2005 paper that explores the customized IBM PowerPC processor designed exclusively for the Xbox 360 video game and entertainment system.

The Xbox 360 CPU in Numbers
For the first time, we get full details on the transistor count and other technical data regarding the Xbox 360 CPU. The paper reveals that the Xbox 360 has a single chip with 165 million transistors for its CPU that is a three-way symmetric multiprocessor design, in contrast to the PlayStation 3 Cell processor that has asymmetrical cores. The three cores found in the Xbox 360 CPU are the highest frequency PowerPC cores currently available, running at 3.2GHz.

The three PowerPC cores inside the CPU are identical, except that they are physically reflected through the X and Y axis. Each of the CPU cores is a specialized PowerPC chip with a VMX128 extension related to (and partially compatible with) the VMX instructions found i in the G4 and G5 CPUs used on Apple computers.

The three CPU cores share a 1MB Level2 cache and each one has 32KB each of data and instruction Level1 cache. The chip's front-side bus/physical interface has a 21.6GB/second bandwidth, and runs at 5.4GHz. The high frequency clocks are generated on-chip by four phase-locked loops.
The paper also explores each PowerPC core design, the vector units as well as the data and instruction caches, the front-side bus and the debug features. To read the entire paper, click here." [link]