Whether or not you want sata express is kind of up to you. If the price is reasonable, i think theyre worth having, otherwise i don't think i'd overspend for it.
If you put 3k ram in a mobo it will just run it at the speed that it can, unless you overclock it.
Voltage matters in the sense that it needs to be compatible with the motherboard. otherwise voltage does not really matter. chips that are clocked higher tend to need more voltage to be stable, so that is why you will see more voltage on faster ram.