Your CPU and GPU are more important than the difference in RAM speeds

So basically a faster CPU and a GPU that can cope with the hardware and settings you want to view games at. The RAM on your GPU card will help govern the viewing of the game whilst the CPU handles the speed. Your RAM is speed very unlikely to make ANY noticable difference. Anothyer bottleneck for speed is your data storage. An SSD will outperform a normal HD and teh difference in speed between those is massively more important than differences in RAM speeds.

So long as your new motherboard is compatible with the RAM you already have [you can usually check that online] you can just insert the RAM and run it at its default speed.

OP: Depends on your motherboard. Plug in the most powerful CPU you can. That's the quickest and cheapest thing to do.
I have a 3rd gen Intel, bought it back in 2012 with an i5 3550 and a Z77 motherboard and 2x 8Gb 1600MHz DDR3. The best CPU for that socket and chipset was i7 3770K so I plugged that in, and did not have to change anything else. About 15-25% more performance, maybe. The more threads help a lot when you run background tasks. So does the higher frequency (i5 3550: 3.3/3.7GHz, I ran that at 4.0GHz, the 3770K is 3.5/3.9GHz and I run it at 4.2GHz).
Of course you will want to run the whole game (base game + workshop items, mods) from SSD.

My game starts stuttering and at around 60-80K inhabitants if I remember correctly.