Technically, yes, you can compare the Gamecube's graphics to the Xbox. The Xbox has better *actual* numbers, but none of their games shows that it's actually significantly better in any respect. Pull out a great looking Xbox game and there's a Gamecube game to counter it.
And the Xbox and Gamecube games that are also on PS2 (sports, mostly) look the same for those two but both noticeabley better than PS2, which is just technilogically behind.