I have 8 GB now, so the thought of 16 GB seems great. And I will have two open slots to add memory later if I feel I need it. Being able to add memory is a huge advantage of building my own.
That could be a problem. If you install 1 board, that is a kit of 1. If you install two boards that were packaged together, that is a kit of one. If you install two separate boards that were packaged individually, that is a kit of 2. If you mix kits, even from the same manufacturer, even with the same part numbers, even with the same chips, you could have compatibility problems. Maybe it will work fine, maybe it won't. I'm sure that certain boards are more sensitive than others. Chances are that the faster the stick is the less likely that it will work. So, 2133 may work, but 3400 will likely not.
My reasoning is that if you are going to install 16GB, then it is probably better to install 4x4 than 2x8 because all four slots will be populated.
Here's a response to some problems ASUS Rog users were experiencing:
http://rog.asus.com/forum/showthrea...e-memory-kits!-The-meat-and-potatoes-overview
It depends on the mobo and the OS - some will have no problem mixing kits, others will only have problems. (Back in the old NT4.0 days if you mixed kits you were certain to get BSODs. The fix was to install all-the-same-memory (manufacturer, density, chip timings, circuit boards - they all had to have the same part number and then they had tro be checked that they were all identical.) At work I'll get machines with 2x1G sticks and a stray 256MB stick. I tear out the 256MB stick and throw it in the trash. Many times the machine comes in because it won't image, other times because it BSODs in the middle of the day. I check the mobo capacitors and replace any that have pop-corned, then run memory diags before trying to image.)
Yeah, you could install one 8G stick today, maybe even a kit of two 8G sticks. A year from now you may want to see if another kit from the same manufacturer will make the machine run smoother. It may; it may not. If it doesn't you may be able to send it back. But what then? Chances are you'll try another manufacturer. And the same thing happens. You'll try shifting the sticks around, to see if that works. Or you'll just take out the two 8G sticks and throw in four 16G sticks. I know that I went that route with my G5, where I started with two 2G DDR 4200 sticks, went to eight DDR 5300 sticks, then went to eight 2G DDR 6400 sticks. And yes, there times when I had mixed speeds in there. Seeing that each time I increased my memory, and/or upgraded my video card, the machine got smoother. Before I built my present PC I was thinking of upgrading to 32GB. I was still running OSX 10.5.8, but that hardly mattered. What concerned me was the long load times when having 1, 2, 3, 4 dozen tabs open at the same time. Then it took forever. So, when I built my system I had to decide whether to go Z97 or Z170. At the time Skylake was having many problems, so I went with Z97 and the max mem of 32GB (1 kit). When using HDDs programs take about 2 seconds to fully open but I can have two or three browsers open at the same time, each with a dozen tabs open and I have no problems.
My concern is always SSDs. IMO, one should have the max amount of memory the mobo can take so that there is no disk thrashing, no disk swapping, so that one can set up VRAM to run a browser, for example, so that the browser caches don't run on the SSD but rather on mobo memory (NAND).
So, just like me, you may start thinking that you can add 16GB later, but chances are that if you start with 2133 now you'll want 3200 a year from now and 4200 three years from now (after the prices have dropped sufficiently enough to be able to afford it. You can get DDR4 4200 now - it's just that it costs about $500 for about 8G (4x2G?) (or was it 16G (4x4G)?)) Regardless, it's expensive. So m suggestion is to up the speed and bite your lip as you max out the memory. Five years from now you will probably move on to another machine, and will probably relegate this machine to Linux. (I can remember when having 4x512MB of RAM was a lot in Linux.) And I can also remember when it took less than 6 seconds for my Apple SE30 to boot and open a blank Word document. So, it's all relative - you may not need that mem now but chances are you will in the future. Do you save now and spend in the future? Or do you spend now and save in the future? I suggest you spend twice as much now and save twice as much in the future.
Some light reading:
http://www.techradar.com/news/compu...ram-10-memory-kits-reviewed-and-rated-1085281 Has some insights. Who knew that heat spreaders were just hype? Skylake RAM is rated at 1.2V so it's not exactly a space heater. MY DDR3 is rated at 1.65V and I'm not about to crank up the voltage to try and get more performance out of it. If I want more performance I should have bought faster RAM in the first place. [As a few sites have said, the internal speed of the CPU has marginalized the need for the fastest speed (throughput/bandwidth). But not for max RAM. That's why some guys will stick with X99, so that they can run 128GB of RAM. (I'm no Superman who makes money from his machine. But the guys over at DreamWorks probably have max RAM running on their workstations.)]
http://www.lifehacker.com.au/2010/07/what-are-the-rules-on-mixing-two-different-types-of-ram/ DDR3 article.
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Here's JohnnGuru's review of the Corsair CS550:
http://www.jonnyguru.com/modules.php?name=NDReviews&file=print&reid=366
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If you look at your QVL you may notice that some of the 2133 is more expensive than the 2400.
http://www.crucial.com/usa/en/compatible-upgrade-for/Giga-Byte/ga-z170x-ud5
I know that mem prices seem expensive. Paying $250 for 64GB may look ridiculous, but I paid that for 32GB 7 months ago. I wouldn't be surprised if a year from now you can get 64G for $150 - probably because everyone will be buying 4200 RAM for $350. Today you'll be paying $248 - $264 for Crucial 4x16 kits. Go with another manufacturer and you can probably knock off $40. But it may not be as good as Crucial, or Corsiar, or Kingston, etc. & et. al.