While that is true, it is less useful than it sounds for single-core performance. All modern OSes I know of use RAM pages, which allocate* blocks of the sticks of RAM for usage in program. If you need an address outside the currently allocated space, it performs what's called a page fault (not an error, more like a garbage collect) and allocates the needed block. This means the program has to wait until that block is allocated.

The only case where dual-channel memory helps in this scenario are TWO programs getting a page fault at the same time. Not just two programs using memory, but two programs happening to need non-allocated memory at the same time.

On the other hand, as far as page fault time usage is concerned, Linux uses about 30% of its CPU time dealing with page faults. So, the performance difference for multiple RAM-heavy threads will be noticeable, but it won't be double. It might be 8-10% faster in that specific scenario.

One stick of 8GB RAM is cheaper. Is the extra cost worth the slight bit of extra performance, and the inability to do a partial upgrade later? (ie buy another 8GB stick and get 16GB total, rather than buying 2 8GB sticks later to swap out the 2 4GB sticks)

*in this case, allocation refers to either copying the block of RAM into the CPU cache, or letting the MMU do some magic. It's not referring to malloc() or new() stuff, although those can easily cause page faults.

That one is only a Micro ATX, and I need an ATX motherboard. The H81 Pro looks like it's an ATX version of the H81M, so I might consider that one.

Partly for my education: why would one prefer an ATX motherboard over micro ATX? I know you'd have to switch cases too, but is that the only tradeoff?

Any board will fit in an ATX case. In a mATX case you need a mATX board or mini ITX. For Mini ITX, only mini ITX boardd will fit. But if you have a larger case, then don't limit yourself to a smaller board, IR you won't be able to expand the machine.

If you have a full size case, take the full size board.

Besides, it's easier to install smaller motherboards in bigger cases. The screw holes will still line up, and you'll have more room to work with.

In my experience by the time you upgrade you want to replace the whole thing anyway, but then again, I am a cheapskate. I say go for the H81M.

Why did you downgrade the CPU from 4150 to 4130? Although prices fluctuate, it seems that you can get the 4150 for the same price or even cheaper than the 4130 on average.