A couple things:
-The size of these screenshots are hardly an indication that the game will be HD
-Current Wii games render at 640x440ish according to Quaz51.
-The hardware in the Wii might not allow for HD output. I'm not talking about having the horsepower to run in HD, I'm talking about having a DAC (digital to analog converter) capable of handling HD output. So even if you just want to output a simple HD picture, the Wii might not be able to do it.
-Rendering to the GDDR3 RAM is slow I believe. At least that's what I gathered from reading the Beyond3D thread about the Wii.