you generaly want to keep ram MFG and specs the same when adding sticks. that said i have added mis-mathced pairs before and gotten away with it but more often than not i need to tinker with settings to make them function right.
your 12GB soultion should give you dual channel still but some MB are finiky with that so you may only get single. i would buy the 2X4 and try it with your current and see what happens. that said... if you can support 1600Mhz sticks then buy 2X4 1600 and dont use the current ram at all.