Wii is roughly 4 times as powerful as a PS2..
Graphics do matter, but the most important thing for a game is to have good gameplay or, more simply, to be fun..
So whether it's using a D-Pad and 2 buttons as in Pokemon Blue or using pointer controls in Metroid Prime or The Conduit or simple motion based controls like in Wii Sports or Grand Slam Tennis or a combination as in MadWorld and Super Mario Galaxy.. The fact is it's not graphics or controls that are the main thing.
It's the games!
At the moment, the PS2 doesn't have much hope at touching Wii as the main market that the Wii is going for are interested in the software such as Mario Kart, Wii Fit and Wii Sports that wouldn't be as fun on other platforms. The easy nature of using the Wii as opposed to the PS2 (TV style controller) etc..
Also before anyone says that it won't work because the GameCube couldn't get an audience below the $99 price range, have a think about why that may be.. It's because the GameCube didn't have the games that the consumer wanted - PS2 and to a lesser extent, XBOX had them.
Now Wii has the mainstream games and PS2 does not..