Okay, mini-wall of text.
1. Going with a 4x4GB kit will actually not improve performance. It's completely identical - actually worse for this platform in some ways. It puts extra stress on the memory controller onboard the CPU(albeit by minor amounts), and since you have more sticks you have higher chances of developing bad RAM over time. Go with 2x8GB - although Corsair Dominator Platinum is way overpriced unless you want the asthetics.
http://ark.intel.com/products/88184/Int ... o-3_60-GHz
If you scroll down to memory specifications, the Max # of Memory Channels is 2. It is 2 for all chipsets on the LGA1151 socket. That means that RAM will run in dual-channel whenever possible, and therefore 2 sticks is the most you can take advantage of. 4 RAM slots means that you can just add another 2 sticks in the future if you ever want 32GB of ram.
2. While the CX lineup of power supplies isn't amazing, they'll work for the budget. You're going to be barely pulling 350w at stock speeds, so I don't see any real issue. Of course, it'd be nice to grab one with higher efficiency and better build, but plenty of builds have used CX500 and CX600 series power supplies with little to no issue. IMO, since you have it, it's not worth returning.
3. SSD makes your overall computer usage feel super snappy. I'd highly recommend one. ~250GB if possible, but ~120GB is definitely doable. I have a 120GB in my own system, backed up by a 1TB WD Blue for storage. You still have around 50GB on your SSD after windows gets installed.
Some more things about the build:
- You've got a locked CPU with a motherboard and cooler that are aimed at overclocking. You won't really be able to properly overclock the CPU unless you grab a K skew - in this case it's the i5-6600k. Also, the H100i GTX is pretty noisy. I'd highly recommend grabbing a large air cooler like the Noctua NH-D14/NH-D15 or the Phanteks PH-TC14PE since they perform similarly to 240mm radiators but already have larger and extremely high quality fans attached. If you aren't overclocking, a H170 or B150 board with stock cooling would be a much cheaper route to go.
- As I've stated before, It's really not worth going for Dominator Platinums. Many cheaper kits from Corsair, PNY, G.Skill, or Kingston will perform the same at much cheaper prices. All ram sticks should come with a lifetime warranty, so there's no real reason to go for the Platinums unless you really want to pay for the kit's asthetic.
- Consider the R9 390 over the GTX 970. The R9 390 performs around 5-10fps faster in most titles than a GTX 970, and won't stutter when you pass the 3.5GB VRAM limit of the GTX 970. The last 500mb of the GTX 970 is a partitioned section of slow-accessing memory due to the way the GPU has been cut down from the GTX 980, resulting in a lot of stuttering once you pass that limit. Your case has plenty of space to accomodate the extra heat that the R9 390 puts out, and your power supply still has plenty of headroom.
//after typing this I just remembered you have the 970 already. Eh. If you have to pay for return shipping, then just keep the 970. It still performs well enough.
- I personally wouldn't get the deathstalker. Spend the extra money on a mechanical. Just find out what type of switch you want - it'll vary between Blue, Brown, and Reds mostly. I don't recommend the newest lineup of Razer keyboards since they're replaced the German Cherry MX switches with Chinese Kailh ones and beefed them up with marketing. A Corsair Strafe RGB would be a good pick. The deathadders are fine though. There was a 50% off sale yesterday on all razer products though - you missed it
- Also, headsets are crap. Period. Grab a nice set of headphones and a desktop microphone. If you still really want a headset, I'll recommend the HyperX Cloud or Cloud II. These headsets were made in partner with QPad, a reputable headphone design company.
And finally, some advice about purchasing parts:
Purchase parts over a 1-3 week timespan at most. If you buy them one by one over a span of a couple months, you just purchase parts earlier in their lifecycle. Doing so means that you pay more for parts that might actually become outdated by the time you get all your parts in and built.