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I agree with Daveway on checking out barefeats, especially for more real-world-ish tests. For an attempt at pure benchmarking, see Xbench, especially their user-submitted results. Macworld also publishes its own benchmarks with its Speedmark, but be aware of different versions used for older systems.

There's also MacSpeedZone, but I don't know how frequently it's updated.
 
Yeah, Daveway and mcgarry have them covered but just remember that all these benchmarking sites take into account different aspects of computing so an alternative might be to give Final Cut Pro a run on some machines at a store to see how snappy it feels. Of course, the benchmarks are a good idea too though.
 
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