if Nintendo do use a Tegra 2 looks likely, I think they will probably go for a lower clocked variant so they can maintain battery life but even then they will be nearing NGC and hence the Wii in graphical power. actual performance will depend on the speed and amount of the ram they use though.
P.S. the Tegra 2 isn't gust a GPU it's a system on a chip design incorperating one or two ARM cores (like the one in the DS but more advanced) and GPU components aswell memory controlers etc
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