Macbook Pro Order of RAM Loading
Typically you want to install memory in pairs (i.e. two 8 GB "sticks" or 4 GB "sticks") to make use of something called the dual-channel memory interface. What that in essence means from your perspective is that the computer can use two memory modules at once, one in each "channel," if they are matched. That gives better performance than using one and then the other.

The way Mac OS X works, your computer will use all of the available RAM when it can to cache things that it might need later--even if you don't need them in RAM right at the moment. The price of RAM what it is today, I would try to order another module of the exact same type as your 8 GB.

Source: http://support.apple.com/kb/TA38415?viewlocale=en_US&amp;locale=en_US