The uC is going to have to read one quadrature encoder and do PID control for a motor, and read 16 sensors, which are analog signals that are multiplexed. It will also have to process those sensor signals in a small way, as well as communicate over the UART with the PC for control commands and with one other similar circuit for control cooperation.
Currently, the Axon I have is doing the encoder/PID thing for 4 motors (it did 7 once :-P) and the sensor reading for 16 sensors, but I haven't put the processing onto it. I don't imagine it'll be all that advanced for it -- the processing is something like take the average of all the sensors, then compute the numerical derivative, then monitor that signal for a while.
The '644s are actually just a test bed. I want to learn to make the circuits, then test out the ideas on the 644s (mainly because I have two 644s sitting on my desk, and they are PDIP so I can toss them into breadboards). I was thinking of using ATMega128's, the 44 (or 40?) pin surface mount version, like on the Axon Mote. :-P Did I mention that the entire circuit, including all IO and communications peripherals, has to fit on a 1"x1" square?