I have found that everytime someone says the Wii is as powerful (or less powerful) than the XBox that they tend to be the people who are least qualified to comment on the power of any system. Back in the day many developers believed that the Gamecube was the more powerful system of the two being that (at the time) PowerPC processors were far more powerful than the x86 architecture at the same clock speed (consider that a modern x86 processor at 2 to 3 times the clock speed of the original XBox's processor is 10 to 20 times as powerful), and the Gamecube's GPU could handle a similar number of polygons in real world conditions with more texture layers and effects ...
The (unfortunate) problem was the Gamecube's GPU used an older design and incorporated pixel combiners (the TEV unit) rather than the newer (and rapidly becomming GPU standard) programmable pixel/vertex shaders. To make matters Worse the way the Gamecube sold few developers focused attention on developing games that took advantage of the hardware; the PS2 was dominant so every developer saw value in exploiting the hardware, the XBox was so close to a PC that most PC developers put effort into getting good performance out of the XBox (to port their game), but the Gamecube mostly got third rate PS2 ports.
The interesting thing is that the developers who focused attention on the Wii were able to produce games that stand out as some of the best looking games of the generation; Factor 5 was able to achieve performance from their launch game Starwars Rogue Squadren 2: Rogue Leader that surpassed the performance of most other third party efforts on the Gamecube, and then they surpassed that level with the sequel, and Capcom's efforts with the Resident Evil games (in particular Resident Evil 4) were the most visually impressive games of the generation.
Now, the problem is we can't really evaluate the Wii because we don't know how Nintendo changed the hardware. Based on analysis of the die sizes of the Hollywood and Broadway processors they are (roughly) twice as large (in transistors) as the Flipper and Gekko were, we know they run at 1.5 times the clock speed of the Flipper and Gekko, and we know that they have far more (and far faster) memory than was available on the Gamecube.
When you consider that the real-world performance of the Gamecube allowed it to render more polygons than there are pixels at 480p, with more texture data than can be displayed at 480p, and have (for its time) advanced lighting and material effects and think that (by all measures) the Wii is at least 1.5 times as powerful (potentially closer to 3 times as powerful depending on the changes made to the processors) it should be able to do some pretty impressive things.
The fact is that we have started to see some of these impressive things with games like Super Mario Galaxy and The Conduit ... and over time (as developers start focusing on taking advantage of the Wii for maximum impact) we will start to see things far above and beyond what was possible on previous generation hardware.