UPDATE: A poster has correctly pointed out that IBM previously announced the Wii U will use a POWER7 chip, not a PowerPC one. While this casts doubt on these specs, the low-end variant of the POWER7 is four-core and clocked at 3GHz.
Since the Wii U was announced, we've had vague morsels of information deposited into our brains regarding its actual specifications. We now have an unconfirmed report of the console's potential hardware configuration in terms of megabytes and gigahertz.
WiiUDaily has apparently been in contact with a Japanese developer who was happy to spill specifics. On the CPU side, we should expect a quad-core, PowerPC chip clocked at 3GHz, "very similar" in design to the Xbox 360's processor. Memory-wise, 768MB will be shared between the CPU and GPU.
Both claims are gently supported by earlier statements from Ubisoft that the Wii U will have a "multi-core" processor and a "large memory capacity". Sadly, the developer WiiUDaily spoke with had nothing of import to deliver regarding the GPU, other than the fact it's 45nm and designed by ATI.
For comparison purposes, the Wii has a 729MHz, single-core CPU and 88MB of shared memory. So, yeah, with these specs it kind of destroys Nintendo's current little white box. Not that speed and storage matter to a solid gaming experience -- the Wii has shown that you don't need grunt to shift ridiculous amounts of units, as long as you don't care about third-party titles.
Other than WiiUDaily's source, there's not a great deal to back up this information. When you consider the hardware is probably yet to be finalised, and developer consoles tend to have different specifications to retail models -- additional RAM is quite common to give programmers and artists some extra space to play with -- it's best to keep this in the rumour folder for the time being.
Wii U has quad core 3GHz CPU, 768 MB of RAM [WiiUDaily]