Looks like you can set up memory for 1600 choosing: 1) XMP which makes your CPU slower, or 2) manual o/c which overclocks your CPU, too.
Default BCLK for i5 is 133, You probably set XMP profiles on. By default it should have been 133.
CPU multiplier is 20 by default. So it gives you correct CPU speed 20x133=2.66 GHz for your i5 750.. for now it's= 2.56 GHz.
Once you correct BCLK, memory speed should achieve its nominal 1333 speed (133x10 which 10x is taken probably from System Memory Multiplier which is set to Auto now.. you can ammend it by hand). If you want to do it by o/c way - set up BCLK to 160, but turn XMP off.
With the P55 motherboards the CPU determines what memory speeds (via the memory multiplier) are allowed.
With an i5 750 you cannot run RAM any faster than 1333 with effectively OCing your computer or making trade-offs. You can often turn on XMP in BIOS, but this changes your BCLK which also affects your CPU speed. It will likely, as a side effect, end up turning off functionality such as Turbo, EIST, sleep states. You can usually turn most of those back on but you have to specifically do it. But Turbo will be lost unless you OC your CPU. This differs from motherboard to motherboard.
To provide a little more detail on why memory multipliers and XMP affect your CPU, we need to discuss how the RAM and CPU speeds are determined. There are 3 important settings: BCLK, Memory Multiplier, and the CPU Clock Ratio.
RAM Speed = BCLK x Memory Multiplier
CPU Speed = BCLK x CPU Clock Ratio
By default, the 1156 socket chips run with a BCLK of 133. The CPU and RAM attain different speeds by using different multipliers.
The Max Memory Multiplier that is available to the i5 750 chip is one that allows RAM to reach a speed of 1333 MHz at default BCLK. Doing the math, this is basically 10 (some boards treat it differently, but fundamentally it can be thought of as 10) because 133 BCLK x 10 Memory Multiplier = 1333 MHz.
The Max Memory Multiplier that is available to the i7 8xx chips is one that allows RAM to reach a speed of 1600 MHz at default BCLK. Doing the math, this is essentially 12 because 133 BCLK x 12 Memory Multiplier = 1600 MHz.
The i5 750 runs with a default CPU Clock Ratio of 20. This gives it it's default CPU speed of 133 BCLK x 20 CPU Clock Ratio = 2.66 GHz. When XMP is turned on for 1600 MHz RAM, the BCLK is usually changed to 160**. Remember, the max memory multiplier available to the i5 750 is 10, so to hit 1600 MHz the motherboard must change the BCLK to: 1600 / 10 = 160.
(The i7 860 runs with a default CPU Clock Ratio of 21. This gives it it's default CPU speed of 133 BCLK x 21 CPU Clock Ratio = 2.80 GHz.)
As a result of this BCLK change, the CPU speed changes. If the CPU Clock Ratio did not change your CPU would be OCed to 20 x 160 = 3.2 GHz. Most (if not all) motherboards would deem this too dangerous to allow when a user only flips the XMP profile. So, the boards drop the CPU Clock Ratio in response to the raised BCLK. Based on what I've seen around the forum, most boards will drop the CPU Clock Ratio to 17. This means your CPU is running at a speed of 17 * 160 = 2.72 GHz.
I don't know how Asus or others handle this, but Gigabyte treats this change as an OC. If you have left other settings as is, they will disable Turbo, EIST, and Sleep States. These functions can be turned back on, but you have to go in and flip the settings from "Auto" to "Enabled". "Auto" means the motherboard decides whether or not to allow these functions. By setting it to "enabled" you ensure that they are always available.
Of course once you have XMP turned on and your BLCK gets bumped up to 160, if you enable Turbo it will run your machine even faster. When Turbo kicks in, the CPU Clock Ratio changes. For example, with one core of an i5 750 active Turbo might raise the CPU Clock Ratio as high 24. This gives a stock CPU a max speed of 133 BCLK x 24 CPU Clock Ratio = 3.20 GHz. But with XMP on and a BCLK of 160, your new max speed is 160 x 24 = 3.84 GHz. You'll want to test your system for stability running at these settings. Keep an eye on V and heat.
Putting this all together, at default/stock for an i5 750 we have:
BCLK = 133
Memory Multiplier (effectively) = 10
CPU Clock Ratio = 20
RAM Speed = BCLK x Memory Multiplier = 133 x 10 = 1333 MHz
CPU Speed = BCLK x CPU Clock Ratio = 133 x 20 = 2.66 GHz
When Turbo kicks in (for example, let's say it is at it's max):
BCLK = 133
Memory Multiplier (effectively) = 10
CPU Clock Ratio = 24
RAM Speed = BCLK x Memory Multiplier = 133 x 10 = 1333 MHz
CPU Speed = BCLK x CPU Clock Ratio = 133 x 24 = 3.20 GHz
With 1600 RAM and XMP on**:
BCLK = 160
Memory Multiplier (effectively) = 10
CPU Clock Ratio = 17
RAM Speed = BCLK x Memory Multiplier = 160 x 10 = 1600 MHz
CPU Speed = BCLK x CPU Clock Ratio = 160 x 17 = 2.72 GHz
Turbo will only work if the CPU Clock Ratio is set to 20. So if you turn on XMP, and you enable Turbo and change the CPU Clock Ratio:
With 1600 RAM and XMP on, when Turbo kicks in (for example, let's say it is at it's max):
BCLK = 160
Memory Multiplier (effectively) = 10
CPU Clock Ratio = 24
RAM Speed = BCLK x Memory Multiplier = 160 x 10 = 1600 MHz
CPU Speed = BCLK x CPU Clock Ratio = 160 x 24 = 3.84 GHz
**It's possible when you turn XMP on some boards may not change the BCLK to 160 and the CPU multiplier to 17 and Turbo off.
