Amazingly, after the few modifications, required by ATmega644, of the SDuFAT and Wire libraries, it worked from the first try.
Following are photos of the assembled and partially encased Wise4Sure project. It contains the LED display attached to the plexiglass panel, and the Wise4Sure board plugged in the back of the display. Power is provided through either FTDI or USB connector.
The 4KB RAM of ATmega644 should allow for more complex applications, including IR control, wireless (XBee) communication, networking (Ethernet), X10, wave playing etc. Really looking forward to the expanded capabilities. (With ATmega328, I was barely able to combine reading from the SD card with outputting on the LED display.)
Next step would be the design of a proper PCB.