After doing a lot of research, I learned that a memory's frequency needs to be compatible with what the mobo can handle. But is there any compatibility that needs to be accounted for regarding the cpu? I'm asking because I was told that the i5 processor only runs at a 1066 Mhz speed so if I were to use a 1333 Mhz memory it would be downgraded to 1066Mhz automatically. I had originally thought as long as the mobo could handle the 1333Mhz it didn't matter what the processor frequency was. Any clarification on this would be great! Thanks in advance.
No no, it's not like that to equal memory speed and processor FSB. 1066 Mhz memory is not equal with 1066 processor FSB.
If quad-core processor has 1066 Mhz FSB then it's 1066/4 = 266 Mhz FSB per-core.
For double data rate (DDR) 266 x 2 (Double Data rate) = you only need 533 Mhz memory speed. So your memory will approx run at 533 Mhz if you set the ratio to 1:1 at BIOS.
To use all 1333 memory speed, you need to overclock the processor's FSB to 1333/2 = 667 Mhz per core. Well that means you have to Overclock insanely to achieve that 667 Mhz FSB per core.
Actual speed is 266 Mhz, and you need to overclock it to 667? That's impossible. The system will be engulfed in flame because of heat if you do that.
So you must run the processor speed at the ratio of, maybe 5:8 of Processor : DRAM speed. This is the reason why RAM with 1066 has virtually no performance gain with 1333 too: Because actually it's already bottlenecked with processor's FSB.
The memory itself is actually not downgraded from 1333 to 1066 like you said, but because of bottleneck of processor's FSB it cannot reach such high FSB, so the memory will still run at 1333, but the processor can't use it all.
^ FSB works differently to BCLK.
With BCLK, normally you have even memory multipliers, like 8x, 10x & 12x most commonly. (Some boards support lower and higher multipliers too.
BCLK x CPU Multiplier = CPU speed
BCLK x Memory Multiplier = Memory speed
By default, most if not all DDR3 RAM runs at 1066 MHz, 1.5 V @ stock settings. Officially, Intel only guarantees that the Core i7 supports 1333 MHz, however, it can run past that quite a lot. Some motherboards support 2200 MHz+, others just 1600 MHz or so.
There's not much difference between 1066 MHz & 1333 MHz frankly.