The nice thing about the Gigabyte board is that it doesn't assume monster CPUs (i.e. it's not an 'extreme gamer' rig) so the CPU only has a single 4-pin ATX12V connection, and the chipset itself doesn't consume much power. This leaves the lion's share of the PSU output for the GPUs.
Why is that a good thing? 8-pin = lower current draw = higher efficiency.
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I've always been somewhat suspicious of the 'extreme gamer' boards like the Big Bong and other similarly risibly named examples. They're all built to expect some 'extreme gamer' to bolt in an overclocked 6-core 200W crazy beast CPU, and the northbridge is built to run the fastest RAM in vast quantities - the coolers on some of these logic boards' northbridges are like GPU heatsinks of yesteryear...
Your Gigabyte board is capable of handling 120W TDP CPU. There is no 1155 socket CPU that uses 200W. While Gigabyte might not be targeting the extreme user it must still meet Intel requirements and Intel requires an 1155 MB be able to power 120W TDP CPU. The 1155 socket has no nothbridge and the southbridge doesn't have that high of a power consumption. Most of that "extreme cooling". is bling bling which sadly does sell.
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What I'm getting at is that the 'extreme gamer' logic boards *could* eat a LOT more of your PSU output purely to run itself, whereas a simpler, lower power board, with a nice low-power bottom-end CPU and slower, lower capacity RAM, would leave more of your PSU's rated output for your GPUs.
The CPU used doesn't affect the MB power draw.
Lower capacity RAM doesn't affect power draw unless loaded.
The memory controller is in the CPU and it is capable of 4 slots the board just happens to have two. No different to the CPU than a board w/ 4 sockets and only 2 are filled.
The reason to get a low end mATX board is to simply reduce costs. Why buy what you don't need right? Still I like an 8-pin MB connector especially power hungry GPU. Otherwise you are driving a lot of current over 2 small wires (2x 12V, 2x ground in 4-pin connector) and that increases power loss.
