There have been tests about the benefits of ram. For a while there the results were pretty clear on the 1600Mhz models being the sweet spot in price and performance. It probably still is, but since 1866Mhz memory is more readily available checking for sales you might get it at the same price as 1600Mhz making it the better choice. In any case we are talking about something like 5% performance differences between slowest and fastest system performances maximum so there is no huge differences to be had here.
Your PSU should be fine even if you overclock.
Non modular PSU: all cables come fixed to the PSU, so you can't just put in for example 3 cables, all 10 or so might be connected.
Semi modular PSU: Usually the motherboard connector, EPS connectors ( and sometimes 1 PCIe connector and/or one set of Sata connectors ) are fixed, the rest you can plug into the PSU if you need them. Reduces clutter in the case
Fully modular PSU: All the cables can be detached from the PSU, you can connect only those cables that your computer needs. Generally making for reduced clutter inside the case.
Generally asus makes good motherboards and yes it allows overclocking. Obviously with any overclocking your mileage will vary.
It produced solid overclocking results and received a smart buy award from Tom's hardware here. So should be fine.