Up until now it was generally believed that the Wii U would pack about 1.5 times the punch of an Xbox 360 or PS3, a rumor fueled largely by Nintendo's own assertions of the same. They even claimed as much at E3—trust us, we were there.
It turns out, based on information obtained by Wii U Daily from an unnamed Japanese developer currently porting a PS3 game to the new system, that those estimates may have been slightly generous. The Wii U will reportedly carry a quad core 3GHz CPU, 768 MB of RAM, and an unknown, 40nm ATI-based GPU. This puts it at around the same power as an Xbox 360. At best, it might be a slightly more robust system.
Their source also mentioned that Nintendo's been considering another version of the console with a full gig of RAM, which might grant it an extra bit of brawn. There's still a ways to go until the console's release, though, so consider these specs less-than-final, if you please.