If you mix them together, you are better to set the settings to auto. One thing you need to make sure the RAM is 1600MHz 11-11-11-30 or 1866MHz 11-11-11-30, if it is the 1600Mhz, then they will run in the lower speed/timing at 1600MHz 11-11-11-30. If RMA is 1866MHz, then they will run in 1600MHz 9-9-9-24 after you mix them.
Also when you mix the RAM, the RAM will run in the single channel mode because you have 3x4GB, even though they have a small difference between them. http://www.tomshardware.com/reviews/PARALLEL-PROCESSING,1705-11.html
And there is no difference between 8GB and 12GB. If you don't use the PC for heavy photo/video editting then just get other 4GB RAM. http://www.tomshardware.com/reviews/ram-memory-upgrade,2778-8.html