I have 8GB of G.SKILL Ripjaws Series 8GB (2 x 4GB) 240-Pin DDR3 SDRAM DDR3 1600 (PC3 12800) Desktop Memory Model F3-12800CL9D-8GBRL in my desktop.

I want to bring it up to 16GB. Can I mix the RAM I have with G.SKILL Ripjaws X Series 8GB (2 x 4GB) 240-Pin DDR3 SDRAM DDR3 1600 (PC3 12800) Desktop Memory Model F3-12800CL9D-8GBXL as long as the DIMMs are paired?

generally you can, it depends on the system. there was a triple channel memory system at one point. But if yours is dual channel, then it should work. (the specs are so similar on these, I think it will work no matter which way you do it, you just have to try.)

Usually the manual for the motherboard or the system will tell you which way they want you to do it, but you won't hurt anything by experimenting with it and running some memory tests if you don't have or can't find the manuals.

Even if the DIMMs in a channel are different, the system firmware should be smart enough to use the lowest settings, highest number of wait states necessary to make it work. Of course, you are depending on the ability of firmware programmers to make that work properly. And that isn't the idea setup either.

edit: the specs are so similar on these, I think it will work no matter which way you do it, you just have to try.

-David

Thanks LIH Prem.
My motherboard does not support triple channel. My main concern was if two DIMMS in a channel are different. There aren't many CPUs that supprt triple channel, are there?
I am more concerned whether disadvantage of using different DIMMs in a channel would significantly reduce the advantage of doubling RAM from 8GB to 16 GB.

Crucial.com has great information about what works/doesn't work on specific motherboards. Simple to look up your computer model or motherboard.

Yes, but you have 2 pairs of DIMMs, and 2 memory channels, so you can just make sure you put the matched pairs in the same channel. They are probably color coded on the motherboard. But even if you don't, it should, in theory work, and the specs on those DIMMs are so similar, I'd be surprised if there's any significant difference between them. However, for best results, just match the DIMMs in the same color-coded slots. (the info is almost always in the manual, including supported memory configurations, etc.)

Intel makes some high end stuff that can use 3 memory channels. Take a look at things like the i7-950 for example.

Anyway, I think you are fine here. Let us know.

-David

Put each pair on it's own channel and I would be quite surprised if there were problems.

I would run Memtest86+ on it anyway just to be sure.

You've checked that the MoBo can support 16gb and the OS will actually see it?

This would all be a lot easier if the OP told us what MB he has. Without that info, we're sort of shooting in the dark.

Yes the motherboard does support 16GB
Sorry, I didn't think the name of the motherboard was necessary as my question was very narrow in scope. I understand you are trying to give the best advice. You have no way of knowing what I know. Had my motherboard not been able to support 16 GB, I would not have not asked the question I did.

Since you did ask, my motherboard is Gigabyte GA-880GMA-UD2H Rev. 2.1

I am not an IT guy and there is a lot I do not know or understand, but I am not totally unaware of issues also, so my questions usually are narrow. However, there may be other issues I am not aware of for which you need more specific information.

complete specs of the system are are given in post #48
Advice on a new desktop PC

Thanks. That's reassuring.

Yes, vast majority of CPUs do not support triple channel. More than a year ago, I had read that i7 9xx were the only ones that supported triple channel. Has anything changed since then?

Thanks, I will.

Nice build!

excellent. Take a look at page 16 and 17 of the motherboard manual.

The slots are color coded. Gigabyte makes nice boards at that level.

I don't know about that $18 case though (I guess I always overspent on the cases, and got nicer ones. Though it's been years since I built anything.)

Nope, that's correct as far as I know.

-David

Thanks. At the moment, I am in Capetown, South Africa. I will look it up. I do recall that the slots were colour coded.

Yup. White & Light Blue. Alternate your memory sticks, A B A B, to take advantage of dual-channel capabilities. Also, your mobo supports up to 1333 MHz in native mode. You must use the overclocking features to take advantage of the Ripjaws 1600 MHz. You are going to have to set the memory timings and the voltage in your mobo BIOS. See here.

Apparently, the only difference in the two sets mentioned is some extra timings capability. Otherwise, identical specs and hardware.