Not adviseable , it's always best to use same size and same brand , with any ram .
This configuration seems within the specifications of the board (dual channel). See also:
Intel explicit states in that article that same brand, same timing, same DDR speed is not needed. However same size is required (but you already covered that) and it is important that know that speed of slowest memory module defines the memory channel speed.
If your buying 4 new modules then I would advise to buy from same brand to reduce any risk, but I assume that you want to re-use some memory that you already have.
Should I choose 1333 or 1600MHz SDRAM modules? The difference is $10 per module.
How does the system know what is the SDRAM module speed?
Do I set this SDRAM speed in this Intel DZ68BC motherboard BIOS? How?
Please confirm, 4 DIMM slots:
should work fine, right?
I was alos told, we should have at least mininum two modules to start with in the mb, to use the dual memory technology?