I have an Asus X52JC laptop. I have a 2 GB 1066mhz stick, and would like to add another 4gb. If I add 4GB DDR3 1333MHz memory, will the new memory be compatible with the motherboard and with the old 2 GB memory?
Yes, it will be. The rated speed of modern RAM is simply the maximum clock speed it can support. It will have no problem supporting lower speeds. The motherboard will select the fastest speed that all sticks support.