The Wii is definitely more powerful than the original Xbox, but it's not a big jump. It still uses a single core CPU clocked below 1GHz. You can't base everything on numbers but if you understand the architecture of the chip in question you get a good idea of the performance you can get out of it.
The GC and Wii CPUs are basically PowerPC 750 chips (G3) with added instruction sets and improved FPU. The Xbox used a hybrid Pentium III/Celeron but it was more of a PIII with Celeron's L2. Once you understand this it's easy to figure out performance, or at least make comparisons.
You can get a lot by looking at the numbers, and while Wii beats Xbox in just about everything it would still benefit from having a beefier GPU. I still say the original Xbox can do some things better than the Wii, basically anything that has to do with shaders.







