Well, if you just look at how hardware has improved over the past several decades a "Wii Like" system that launched in 4 years would probably be able to play a game like Crysis at medium detail at a resolution of 720p with a steady framerate of 60fps ... The reason for this expectation is simple, the top of the line GPU from the end of 2008 will be outperformed by a mid-line GPU at the end of 2009, which will be similar to a low end GPU from 2010, which will be similar to a mid-level laptop GPU in 2011, and a low-level laptop GPU in 2012.
Edit: I think you should expect the console manufacturers to all look for more powerful hardware than this, but I wouldn't expect them to really be pushing the limits like Sony or Microsoft did in this generation.
