Sorry know this thread is a little old now.
The resolution of games varies from game to game. For example Mario Kart 8 is 1080p, while Assassins Creed III is 720p. They might have alternate modes and scaling and etc. enabled also. Like someone said above it is overall faster than the PS3 and Xbox 360, with a much faster GPU and a CPU which is around the same performance. So Wii U can typically handle 1080p, and as the system gets tweaks over time it is fixing software issues and its suspected that they either have already or will in the future increase the clock speeds. Not double them like some people have suggested, but a fair rumor is that they will increase the CPU and GPU by 100-300Mhz to add a little boost.
So hope this helps...