Depends on the modules -- First off what are you doing that needs more than 8GB. of RAM ? If nothing then Stick with the 1333Mhz. modules and sell the others !
If you want to keep all 12GB. then the next question becomes are they all the same voltage ? - the system can only run all modules at the same voltage so if they have different voltage requirements you might run into stability problems !
Next question you'd need to find an answer to is will the 1066mHZ. modules run stable OC'd to 1333 Mhz. @ the voltage of the new modules ? If they can run stable at that setting then run all 12 GB. @ 1333Mhz. if they cannot then you need to run the newer faster modules at you old modules speed - in which case you then need to determine if running 12GB. @ 1066Mhz. gives you better performance than the 8GB. @ 1333 Mhz. (again that will depend on what you are doing that needs more than 8GB. !!