"So, the Gamecube was widely considered to be roughly as powerful as the XBox"
Very "roughly" the xbox was actually far more powerful than the Gamecube. It was an ugly huge brick but it was a pretty fast machine. Comparing GC, XBOX and Wii is hard though.
"is that the Wii doesn't have many games that have frame-rate issues while a large portion of the XBox 360/PS3's library has noticeable frame-rate problems"
With the exception of Mass Effects Framerate problems mostly exist in multiplatform games like Assassin's. Uncharted and Ratchet were smooth as silk. Since Nintendo doesn't have the multiplatform problem or games are ported from a much less powerful PS2 they have it easier.







