Are there anything to gain from those chips, compared to the ones on the uno?
Compared to the Uno's 328 chip the 644P chip has double the program memory size, more SRAM, more I/O pins, 2 serial hardware usarts, more user interrupt pins. More timers, I forget?
You get the idea, it's a 328 X 2 chip that still comes in a user friendly DIP package.
Lefty