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.
@dudewitbow Most things I've seen indicate it's an AMD card with an R7xx chip, which is the 4000-series.
Keep in mind that it looks like it's customized with a bunch of eDRAM, which is a large part of what helped the 360 stay relevant as long as it did, so it's not unreasonable to think that the Wii U might be able to keep up with the next-gen GPUs after extensive optimization.
what the wii is trying to do is optimize the price to gpu ratio for its build, with the line where 1080p is met and there's enough shaders in the gpu. I'm going to assume that nintendo doesn't want to make the same mistake sony made with the release of the ps3 and its 600$ price tag at launch. they are looking for the sweetspot in which 3rd party devs are willing to use the wiiu while keeping the cost down to a minimum, for the consumer.