Let me say upfront I do not have an Asus motherboard. I will share my experience with a P55 Gigabyte and you can ignore it if you like. I have the
G.SKILL Ripjaws 4GB (2 x 2GB) CL9 1600 (PC3 12800) F3-12800CL9D-4GBRL. My experience with the Gigabyte board is that if I dropped this in the RAM ran fine, albeit it 1333 MHz. It runs just the same as if I had put in the
CL9 1333 Ripjaws.
You are correct that without OCing your CPU you would not be able to run your RAM at 1600. However, between the two sets of RAM I linked to I would still recommend the 1600 over the 1333. Why? Because other than frequency they are the same. They both are rated at 1.5V, they both have 9-9-9-24 timings, and they are both currently
the same price. The 1600's, when dropped into my motherboard without changing any other settings, run as if they were 1333 MHz. So for the same price you get something that's qualified to run faster, even though you may never run it faster.
I also wanted to clarify the comment earlier that said get 1600 RAM and turn on XMP. It sounds like you may have already seen this elsewhere - but you do not want to turn on XMP if you are not interested in playing around in BIOS or OCing your CPU.
And now I'm going to dive off your original question to provide a little more background here for those interested. This is more than was asked so feel free to skip the rest of this response.
With an i5 750 you cannot run RAM any faster than 1333 with effectively OCing your computer. You can often turn on XMP in BIOS, and many people recommend doing this, but this changes your BCLK which also affects your CPU speed. It might, as a side effect, end up turning off functionality such as Turbo, EIST, sleep states. You can usually turn those back on but you have to specifically do it. This differs from motherboard to motherboard.
To provide 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 (this could differ between boards as well, but in my experience and what's been reported the BCLK changes 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.
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 all a user does is enable 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 (all?) 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 other motherboards handle this, but Gigabyte treats this 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, Turbo 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 Voltage and heat.
Putting this all together, at default/stock 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
With 1600 RAM and XMP on, when Turbo kicks in, assuming it's turned back on (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
**I haven't tested an i5 750 with 1600 RAM and XMP and Turbo turned on to know if this is how it will actually handle the Turbo mode. (If you do run in this mode, please test it and let me know if my hypothesis is correct. It's possible the turbo doesn't ramp the cpu multiplier all the way up to 24. And of course some boards may not change the BCLK to 160 and CPU multiplier to 17.) However, based on my testing with an i7 860, to enable Turbo with XMP on in this scenario you will have to
specifically enable Turbo AND set your i5 750's CPU Clock Ratio to 20. This means that your system will be OCed to a pre-Turbo level of:
With 1600 RAM and XMP on and CPU Clock Ratio to 20 when Turbo and EIST are not in effect:
BCLK = 160
Memory Multiplier (effectively) = 10
CPU Clock Ratio = 20
RAM Speed = BCLK x Memory Multiplier = 160 x 10 = 1600 MHz
CPU Speed = BCLK x CPU Clock Ratio = 160 x 20 = 3.20 GHz
As I said before, you're overclocking now and you'll want to test your system for stability running at these settings. Keep an eye on Voltage and heat. Also, although I used XMP as an example you could accomplish the same thing by setting BCLK, the memory multiplier, timings and voltage by hand.
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