I'd also like to note you're forgetting several important factors, like pixel pipelines and memory latency FSB speed that can improve end results dramatically even without a dramatic increase in processing speed.
A 400Mhz GPU Processor with 20 parallel processing pipelines will often be better than a 500Mhz one with 10
Edit: My point isn't so much to prove that Wii is 'next gen' especially considering it's current gen, like the others. It's more to prove that while most people only think of the processors speed is far from the only factor and while Wii may be similar to processor speed specs to the original xbox it could even still be superior.
Edit 2: Also keep in mind my explanation is overly simplistic on purpose, I really need to have the drive to explain in detail.
