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As it happens, I've decided to go with plan B, moving the tower out to my back porch and running cables under the floorboards through to the front room, taking the noise and the heat out there, and drying my washing faster in the process! It'll also leave space in the tower for extra hard drives, so I can effectively use it as a NAS too.
The cable run through to the front room should be about 5-8m for various things, so should be ok with the use of an active hub for USB.
Just some 1600MHz RAM would be a good start. I noticed mine was running at 1333MHz recently and boosted it back up to it's rated speed and it made a good amount of difference!
That option may be the better one to be honest; I think it can be a faff making windows behave when you swap a motherboard out as it treats it as a different pc. I'm by no means clued up on that though, so it's probably worth a google