Core i7 processor, and considerations of RAM upgrade configurations

Question

According to Intel, the Core i7 can access:
3 Channels of DDR3 1066 MHz memory...
and if this is true for Apple's new iMac 27 Quad cores, wouldn't it be most efficient to install RAM in sets of three (ie 3x4GB) = 12GB total; not sets of two (2x2MB + 2x4GB) = 12GB? And why doesn't the Apple Store offer those combinations of RAM configuration?
This affect my considerations for future purchase. If not then:
If and when Mac Pro go to Core i7, will this configuration of RAM (3x4GB) be applicable?

Answer 1

There are many models of Core i7. The Core i7-860 used in the iMac (late 2009) only uses dual channel memory access. So you will get the best access speeds if you install memory in match pairs such as 2x2GB and 4x2GB or 6x2GB or 2x4GB or 4x4GB. You can mix and match such as 2x2GB plus 2x4GB sticks. These will all be matched pairs and run at full speed.
Now a new innovation of the i5 and i7 processors with built-in memory management is that they can handle memory this is not in a matched pair without degrading the memory performance of memory that is in a matched pair. For example 2x2GB plus 1x4GB. In this example, the 2GB sticks will be accessed via dual channel and run at maximum speed; the single 4GB stick will only use single channel access and will hence run slower but it will still work.
The Mac Pro architecture uses triple channel access when memory is installed in threes. For example 6x2GB or 6x4GB. The system also supports 8x2GB and 8x4GB but in those configurations, with the current Xeon processor and chipset, the memory will all be accessed then in dual channel mode. If your system is begging for more memory, running with dual channel will possibly be faster than running triple channel on a smaller amount of memory. If you are not severely memory constrained, then running the triple channel configurations will be faster.