You should never just buy 1 more stick...
Dual Channel (Quad Channel) is too fast to just pass it, sure you can Triple Channel on Intel but since we are talking about AMD CPU's, those don't have Triple Channel.
Maybe you should separate PC's for people that OC and those that don't, getting a Z87 board and a Intel CPU with a Free multiplier is pretty useless for people that will never overclock, you could also use the boxed heat sink if you don't OC.
I build a PC for my girl that has a Xeon 1230v3 with the boxed heatsink and a Gigabyte H87-D3H just because i know she will never touch the hardware.
A 750W PSU for the Unicorn system is a bit much... If you don't run a double GPU there is no way in hell you need more then 550W~,
a good PSU with 35-40A on the 12V output is more then enough.
I also would never touch AsRock again.