I realize this is outside your 3GHz to 4.2GHz example but, I hear they've demoed nehalem running
air cooled @ ~4GHz 
which theoretically would make it slightly faster then a 4.2GHz penryn running even a two core game (and defiantly faster with software that can use more cores).
Now, if you bring price into the picture the penryn may equate to cheaper performance

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