I've recently built a new computer with an AsRock p67 Pro3 motherboard (with b3 stepping). I'm currently running 8Gb of PC 1066 (1333mhz) RAM on it, but I have another 4gb of 1333mhz RAM from my old PC. The owners manual indicates that I should only run identical RAM in all 4 slots. I've run different RAM sizes in the past as long as each of the channels had the same size (and all channels had the same frequency). I.e. 2x2gb on one channel and 2x4gb on the other channel all at 1333mhz.
Has something changed with the way dual-channel ram reads, or is AsRock just being overly cautious about what kind of memory it wants people to use?
You're not going to damage anything by running different sizes of sticks in the slots. As I understand it, as long as you have matching pairs in each channel, it doesn't matter if the stuff in one channel matches the stuff in another. That being said, I invite someone with more experience with mix-and-matching memory to dispute my claim.
But you definitely want to make sure that both sets of memory have similar timings and speeds at the given voltage (no higher than 1.575v in the case of Sandy Bridge).
I have the Pro3 mainboard and two different-sized 1600 kits (totaling 12GB) that work just fine in dual-channel mode.
Here's the correct placement:
Slot 1 (closest to CPU): 4GB
Slot 2: 2GB
Slot 3: 4GB
Slot 4: 2GB
Since slots 1-2 are channel A and slots 3-4 are channel B you have 6GB in each channel, and it runs all of the memory in dual-channel mode.
You can confirm this if you have updated to BIOS version 1.90 (highly recommended) -- go into the BIOS and the first screen will mention "dual-channel mode" if it's working correctly.
Note: I moved the thread to the Motherboards sub-section for you. Hopefully, we'll have an ASRock section soon!
Thanks for the tips, all. I currently have just been running the 8gb until I received confirmation from some of the pros here on whether it'd work or not. I'll swap in the 2gb sticks as indicated and see how everything works!