The current PS3 is based on an NVIDIA 7800-series GPU, and the 360 is based on a more customized/advanced version of the R600 core from ATi (the chip that drives the Radeon HD 2000/3000 cards). Due to massive amounts of specialized console coding, these are (kind of) able to keep up with the Radeon HD 5000 series and the NVIDIA GT 400 series. (Note: This is an incredibly rough estimate and is by no means 100% correct. In fact, this strays into the realm of wildest guesswork).
That said, those GPUs in those cards are able to "keep pace" with cards 4 generations after them after heavy amounts of specialized coding. We're talking assembly-language level stuff, by the way.
So, with that as a reference, putting a card with a similar architecture to a 4870 in the Wii U *should* be able to eventually look as nice as games that run on the NEXT generation of video cards, i.e. the AMD 8000 series, and the NVIDIA 700 series.
Kind of a fun thought exercise.
*Disclaimer: This estimate is in the wildest realm of guesswork. I've made some assumptions here that may or may not be correct, but to the best of my knowledge, they were at the very least reasonable, the biggest assumption being the point at which console graphics started hamstringing the development of PC games, which was estimated to be 2 generations previous. This estimate reflects only my opinion, you are free to disagree (vehemently if you must) without me shedding a tear over it.