You must be very sure that the clock speed and voltage that the RAMs run at are the same. Do not mix a 1066MHz card with a say 1444MHz for example. Doing so will result in an overall slow down of ALL the RAMs to 1444MHz. i.e. the slower clock timing is always used.
At the same time, different voltage requirements could result in system instability. This is why it is highly recommended that you use RAMs from the same brand, and why people try not to mix the brand of RAMs.
If you mix sizes, your RAM is forced to operate in single-channel mode. For dual-channel mode to work, all of the installed RAM must be the same size/speed (i.e all sticks must DDR3 PC3 10600 or PC3 12800).