We are on the same page in terms of thinking about a board that has RAM and can provide a portion of the image based on coordinates. But hang in there a bit. I'm in discussion with SparkFun about my proposed ARM implementation, and they like it. Could lead somewhere, not sure. I would like to wait and see.
Meanwhile, if we want to make a prototype, I suggest something relatively simple... there is a DIP-based ARM CPU, and DIP-based SRAM, and we could put together something, but it will take a fairly large breadboard to get enough RAM for a 128KB of RAM. But it could be done, I think. The ram chips are 32KB each, so it would require 4 of them.
I have not looked, but there may be DIP versions of the faster 32-bit AVR also. Could check on that...