3-4x is pretty much what the gang here has speculated/confirmed with bgassassin, Ideaman & others.
The leaked dev kit specs from last year are at least on par with that as well.
The GPU & RAM of the Wii U seem to be the strong suits thus far for the system, with the CPU being out of order and not able to be used to the full in 360/PS3 ports.
Yup. On paper we're looking at 3 times more powerful than the 360 going by what's been leaked so far - 3MB eDRAM for the CPU against 1MB, 32MB eDRAM on the GPU against 10MB, 2GB of RAM (with 512MB reserved for the OS) against 512MB.
Mix in a DSP and OoOE CPU and we should be looking at around 4 times more powerful in terms of real world performance by my reckoning.
What's most important is Nintendo's (surprising!) forward thinking in using a GPGPU and what appears to be a similar architecture to the PS4 and 720 meaning that the U should be able to provide up-ports and receive down-ports to and from the PS4 and 720, albeit with eye candy less impressive.
You've also got to factor in that, first party titles aside, the majority of PS4 and 720 titles during the first year of their lives aren't going to outshine U titles by that much due to the majority of development being done on underpowered/underclocked/unfinished dev kits. The U should have 2 years worth of eye candy being superior compared to competing consoles, and once that third year arrives it should be around halfway through its lifespan and developers, being more experienced in working with the hardware, should be able to squeeze more out of the box.
Nintendo have a huge advantage in being the first out of the gate next gen, something that did Microsoft a great deal of good this gen. They and Sony are fools to give them a head start, generational fatigue started setting in a year ago despite the adoption of motion controls breathing new life into this gen for a minority of people.