Apple hardware: ram on MacBook Pro
You ideally want both banks the same, this will allow interleaving which almost doubles the memory access speed.


  In computing, interleaved memory is a design made to compensate for the relatively slow speed of dynamic random-access memory (DRAM) or core memory, by spreading memory addresses evenly across memory banks. That way, contiguous memory reads and writes are using each memory bank in turn, resulting in higher memory throughputs due to reduced waiting for memory banks to become ready for desired operations.


Source : http://en.wikipedia.org/wiki/Interleaved_memory