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I have the following motherboard and CPU: Gigabyte GA-P55-UD3R mainboard and Intel i7 860. And I have the following RAM: KHX1600C9D3K2/4G (2x2GB) and I want to buy this pair: KHX1600C9D3K2/4GX (2x2GB).

The part numbers match except for the trailing "X", and there are some differences. My RAM has 16 128M x 8-bit DDR3 FBGA components but the others have 8 256M x 8-bit DDR3 FBGA components per module.

Here are the datasheets: KHX1600C9D3K2/4G and KHX1600C9D3K2/4GX.

Can I use them together and will they work properly? More general: what part of the specifications of RAM should match to successfully work together?

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TL;DR

Yes, it should be safe to try. It will probably work.

Explanation: what memory will work together?

The requirements for memory to work together are:

  • The motherboard must have enough memory banks.
  • The memory type (DDR2/DDR3/DDR4/...) must be of the same for all banks.

Furthermore:

  • To enable dual speed memory, you should use the same pair of banks next to each other, if your motherboard supports this.
  • The memory voltage may be different for different banks, as long as it's part of the memory type's spec (also see here and here and here)
  • You may mix single banks next to a pair of banks. The combination of a pair and a single bank might not work. Switching the order in which the memory banks are placed on the motherboard can make difference.
  • You may use the same brand (this is just a best practice).
  • The memory CAS timings may be different, but in that case, the faster bank runs at a lower CAS timing speed. In general, it should work, but in specific cases it might not work. (Also see here).
  • The memory frequency may be different, but in that case, the faster bank runs at a lower speed. In general, it should work, but in specific cases it might not work.

In general, also see this great answer.

It does not matter if the RAM sizes are different or if the chip layout is different.

In your case, the must-have requirements are met, so it's safe (but no guarantee) that it works. You should just try it.

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    Effective frequency and timings have to be identical, nominal one's don't. If two RAM modules that are used together have different timings or frequencies, then the faster one is underclocked to slower one's timings and frequencies. So their nominal parameters can be different, but they will be equalized.
    – gronostaj
    Aug 12, 2015 at 10:47
  • @agtoever: +1 Good edit.
    – qasdfdsaq
    Aug 12, 2015 at 12:29

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