You're looking at this with blinders on.
The Xbox360 is a limited application computer with some very specialized design work that makes it good at a handful of things. Most interesting is its parallelization and massive vector calculation capabilities, which have long been the focus of Apple and IBM's designs. The CPU itself is a multi-core chip feeding multiple vector sub-units (ala AltiVec and SSE) that will handle much of the rendering needs of a game. The GPU is a next generation R5xx that uses main memory and a tiny dab of dedicated memory, not a full-fledged commercial graphics card.
A PowerMac is (usually) a dual processor machine with a wider range of applications, much more expansion capability, and a need to address more tasks than the XBox360. It has system overhead in the form of a full-fledged operating system, I/O paths, and other such drains that the console won't have much need of. In addition, the PowerMac is extensible through the addition of programs that don't have to be specifically vetted by Apple, unlike the DRMed console. There's also the small point that Apple makes most of their money on hardware and puts that back into software, while Microsoft does things the other way around and doesn't care if the physical unit of the XBox360 loses them money.
