The Gamecube could apply 8 textures per pass, while the PS2 could apply only one, so, if the clock rate of both were the same, the Cube could apply two times as much textures as the PS2.
If there is one spec where the Gamecube (and Wii) is severely lacking, that would be pixel fillrate. The Gamecube just can't push as many pixels as the PS2 or Xbox (GC 648Mpixels/s, Wii 972, Xbox 932, PS2 2400).
In fact, I think that pixel fillrate constrains is one of the causes of the letterbox mode found in RE4. In a game with such rich textures and high color depth, pixel fillrate would be constrained, so they limited the visible area on the screen. It's supposed to be a 480p game, but taking out the black bars it's more like 400p or something.
In reality, where the Cube stands varies from game to game. Most of the time it sits just between the PS2 and Xbox, some times it comes out on top of the three and sometimes it's in the bottom.