Note that some machines will have two 8GB sticks, others just one 16GB stick. This is mentioned in the Gamers Nexus interview with some Valve employees, who were talking about the difficulty of finding RAM at any price. They had intended them all to be two sticks, but some will come with one because that's all they could source.
Would have been nice from Valve to be transparent about that. Maybe a little warning that your particular batch performs a little less than others.
What sort of difference does dual-channel RAM make? Some people probably want the single 16GB so they can add their own additional stick. Which option is better is not straightforward.
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https://www.techspot.com/article/3066-single-stick-vs-dual-c... - seems like 5-50% change in FPS! Way more of a factor (on some games) than I thought.
Valve claimed in their internal tests it did not meaningfully change performance, which I'd be inclined to believe. The Steam Machine is likely going to be most often GPU bottle-necked, so the CPU performance regressing by even double digit percentages doesn't necessarily result in any change in gaming performance.
You'll note in that techspot comparison, by contrast, they used the fastest CPU and fastest GPU and then still used medium/low settings to really maximize whatever difference the RAM speed would have. Which is a valid test, but it's not necessarily going to generalize to low-end hardware. Like the CPU being limited to 90fps instead of 120fps doesn't matter when the GPU is struggling to hit 60fps in the first place.