That's overall an adequate computer for After Effects.
See this page for information about hardware for After Effects: http://adobe.ly/pRYOuk
The GPU in that computer is not one that After Effects CS6 will use for GPU acceleration of the ray-traced 3D renderer, but the other GPU features should work just fine. See this page for details about GPU features in After Effects: http://adobe.ly/UHGwMC
@Todd - I was wondering the same thing. I've been considering an iMac along with upgrading from CS6 to CC for a while. You recently were kind enough to answer my query about upcoming support for ray-tracing with the previous iMac's GPU. And a new top model with a 4GB card instead of just 2GB is very interesting. With CC's new feature to 'tolerate' unsupported cards, what will happen here? Is it the case that the 780M might help ray-tracing with no guarantees for an unsupported configuration, or rather that it definitively won't? And am I right in suspecting that this card should be great with Premiere Pro from the get-go?
I also notice that from the introduction of the last iMac to the adoption of the 2GB card by Adobe in the white list almost a year went by. Is it likely to take as long this time round?