Sorry for the vague description of this project, it is still in the dream-about-stage
So let's just concentrate on the fast data transfer issue. Thanks for any hints.
This is the problem with answering any of your questions. Without understanding what are you going to do, on a detailed level, how can any advice be given?
You seem to have made the assumption that the ATmega isn't "powerful" enough for your application, whatever your application is. Why is this the case? Are you doing DSP intensive operations in your code? If this is the case, maybe the ATmega/Arduino is a poor choice for this application.
You want to know how to transfer data fast. What is fast? How much data and what kind of data? Is this a stream of data, a burst, in and out, etc?
The more useful information you give, the more useful answers you will get.