Coming from an HDWarriors interview with Manfred Linzner of Shin’en...
“The Wii U eDRAM has a similar function as the eDRAM in the XBOX360. You put your GPU buffers there for fast access. On Wii U it is just much more available than on XBOX360, which means you can render faster because all of your buffers can reside in this very fast RAM. On Wii U the eDRAM is available to the GPU and CPU. So you can also use it very efficiently to speed up your application.
The 1GB application RAM is used for all the games resources. Audio, textures, geometry, etc.
Theoretical RAM bandwidth in a system doesn’t tell you too much because GPU caching will hide a lot of this latency. Bandwidth is mostly an issue for the GPU if you make scattered reads around the memory. This is never a good idea for good performance.
I can’t detail the Wii U GPU but remember it’s a GPGPU. So you are lifted from most limits you had on previous consoles. I think that if you have problems making a great looking game on Wii U then it’s not a problem of the hardware.”