Maybe in imaginary numbers. In real-world performance, not so much. So far I've seen a grand total of 2 games that look significantly better than anything on PS2: Mario Galaxy and Smash Brothers Brawl (and I haven't seen the latter in motion). The rest look no better than the PS2, and frequently look worse. And so far there isn't the slightest indication that this trend is turning around. Having a box on my TV stand that's theoretically more powerful than the box next to it doesn't make a whole lot of difference to me.
If a last-gen console owner wants a graphical upgrade, the Wii simply isn't a viable option. (Not that it isn't worthwhile for other reasons).