I would start by checking out the Intel CPU Overclocking thread and the watercooling thread. You will find lots of info and help in the proper thread.Nice set-up by the way. It's hard to believe you haven't overclocked yet.
For 2 290's and an 8350 if you overclock at all I would get a 1000w PSU. I like Seasonic myself, the SS 1050 xp3 is a super nice unit.For a bit less the RM 1000 is nice, it's only gold rated but would be easily good enough. I also have heard some good things abut the EVGA Supernova's.
I recently had my 290 go bad. I deleted the amd drivers but had to download the GPU driver for my 4690k to make it work. I had no need to install the graphics driver before so it wasn't. Try to boot in safe mode if you can and try this.
All I used was the LAN driver to connect and then when to the Asus sight and downloaded updated drivers. If you plan on any overclocking or even stretching the 4790k's legs a bit I would go with the Z97-pro. It has a 12 phase power delivery where the Z97-A has only 8, plus the heat sinks look so much better and on top of that it is a bit more feature packed. I know on average it's $40 more but you can find deals and $40 is well worth it anyway.