never change a running system, I think I'm fine with my F28, maybe I will upgrade a SSD (even then F28 should be good since I can't find any other infos in the version history, except some problem not booting form SSD) or sell the whole thing again. The memory was so damn cheap that I would make close to no loss in this case. Anyway those 1600 MHz RAMs should run at 1600 and with 1,35V in that model, at least they are officially supported by HP. It even looks like they switch over to DDR3L since the regular DDR3 Modules were discontinued in 2013.
And for your WiFi question: Do yourself a favour and buy a USB Dongle or something like that, I mean all this stuff with internal antennas sucks! Especially when it's in a metal case...even my Smartphones WiFi is lightyears ahead. Preferrably something with a RP-SMA connector (or DIY something, shouldn't be a problem as far as I've looked into this thing everything has regular connector). I have one of those bloody cheap TP-Link TLWN722N, 15 Euro with a good 8 dbi antenna, only downside: no 802.11a but it allows prosmicious mode, packet injection and all those funny things.