It gives you the fastest possible PowerPC mac you can get with Classic backwards compatibility with damn near everything and it will last almost forever as well.

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Do not forget that you need to stick with 10.4 instead of 10.5 if you need Apple's Classic support. As SheepShaver on recent Intel gear is a well working solution I would think twice if I wanted to buy outdated hardware to run the emulation of an even older system software.