I'm curious if my 2 DIMMs are running dual-channel or not and use:
dmidecode -t 17
Type Detail: Synchronous
What does a "Synchronous" type detail mean? Dual-channel?
It means that the RAM sticks are driven by a clock signal — as opposed to (older) asynchronous DRAMs. See the Wikipedia page on SDRAMs.
To find out whether your RAM is dual-channel, look for “Interleaved Data Depth” in dmidecode output. It should say 1 for single and 2 for dual channel RAM.