Hmm, I think if you going to compare the Xbox360 GPU with the one from a Wii, they are both made by ATI but then the comparison stops.
Afaik the Wii GPU is a lot similar to a Radeon X700 (8pipes, 243mhz core, 64mb dedicated gddr3) (*), while the Xbox 360 GPU is a lot like the Radeon X1900 (48pipes, 500mhz core, 512mb shared gddr3).
Huge performance difference, but I have a Wii and all I can tell you is graphics have very little to do with gameplay.
(*) Some say the Ati "hollywood" GPU is based on the X1600 (RV530), which might be possible too as it would make more sense as this is a more uptodate chip with real low power consumption. The Wii only consumes 17W under load so I think a crippled X1600 could be under there bonnet.