I think Halo 3's opening week numbers are safe for the rest of this generation. It's possible that something will generate that much hype in the next few years but perfect storms (combined with massive advertising dollars) don't come around that often.
SSBB might outsell it lifetime though. It's hard to know what the Wii demographic looks like right now with the influx of traditionally non-gaming people buying it in huge numbers. They could be drawn in to it and it might crack 10m or it could just do marginally better than Melee.

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