This is great list, pretty exhaustive. The only one I would add are browser add-ins.
First and foremost Ghostery. It displays and allows you to block "cookies, tags, web bugs, pixels and beacons—and gives you a roll-call of over 1,800 ad networks, behavioural data providers, web publishers and other companies interested in your activity". It really sped-up my browser and it was interesting to see the companies that were tracking my activity.
I would also recommended an adblock extension, either adblock or adblock plus, depending on your browser
If you have an old smartphone lying around and only need your keyboard and mouse to get into Steam big picture mode and/or netflix, you can use something like remote mouse.
There's no reason for it not to accept two different size modules.
The timings and speed will be synchronized, but the mobo will still detect the DDR3 modules as being different sizes.
http://superuser.com/questions/696433/can-i-have-two-different-capacity-ram-sticks
Superuser is renowned for giving accurate responses every time, and this one has multiple that agree with my stance.
I agree. Aside from the sheer giggles, selling it would be the way to go. A kickstarter launched selling them for about $1250 so there is some money to be made
Do you have the product key for your Windows installation?
Once you have your key you should be able to do a fresh installation using a Windows 8 ISO burned to a blank DVD.
Check your temperatures with open hardware monitor: https://openhardwaremonitor.org
You want most Temps below 90°C. Typically 70°C should be your max, but it varies by component and above 90 is where you'll start to have have problems.
One other problem I've seen before could be with your RAM. I can't do it for you right now, but find your motherboard's list of approved RAM and check your RAM kit's part number against the list. It's not always a problem if your ram wasn't tested and approved, but I've definitely had one build where that was an issue and it resulted in a lot of crashing and blue screens.
So I checked the price of the other CPU, the i5-4690 (non k) and on Amazon it wasn't that much cheaper. Some of my previous statements are now incorrect. It appear that the i5-4690k does come with a stock cooler, I was concerned that it did not because in the more recent i5-6600k they no longer package a cooling solution. All in all you could still get a cheaper motherboard an H87 and get either the i5-4690 or the i5-4690k and end up with the same experience. As for the termal paste, the stock cooler has thermal paste preapplied. However, every time you take the cooler off new thermal paste is needed. As for the case, as long as you get a Mid or Full sized case everything will fit.
I personally like how Corsair and Thermaltake Chasiss look but that is personal preference. As long as it is a Mid or Full Tower size everything will fit.
or
Stretch that budget a little and get a good case. I have that case for a HTPC and while it does the job it is a nightmare to work in. You will thank me later, believe me.
Thanks so much for the advice, you're totally right especially about the gpu. I've switched to the PNY KF970GTX4GEPB.
About the PSU, I did a little more digging, and not only is the SG500-LG modular but it's got a 120" fan as it's SFXL form factor. That means it should be as quiet as the be quiet one, including a mode to shut down the fans completely at <40 degrees. Perfect.
Changed motherboard to the Gigabyte H170N-WIFI to save a little too. All the same features, minus the overclocking.
For a cooler I'm thinking of Noctua NH-L9i but I might stick with stock for now to see how it goes.
Amazon is linking to 4 similar items; OP linked to specific models.
Intel Core i7-4720HQ 2.6 GHz / 12GB DDR3L 1600MHz / NVIDIA GeForce GTX 970M
Intel Core i7-5700HQ 2.7GHz / 16GB DDR3L 1600MHz / NVIDIA Geforce GTX 970M
Personally I don't think the 5700HQ is worth the $150 upgrade.
Not a bad build actually, for your purpose. Should serve you well!
An ssd would work wonders for speeding up your load/boot times, and you can pick up 240Gb ones for like £67 off Amazon, or even 120GB ones for like £40.
Solid build, here's a decent 500gb HDD (about as cheap as I could find) if you wanted to double up on your storage for a minor cost increase: http://www.amazon.com/Seagate-Barracuda-ES-ST3500630NS-internal/dp/B000R2GC08/ref=sr_1_4?ie=UTF8&qid=1433516834&sr=8-4&keywords=500gb+hdd
I bought all of the major gaming headsets once a couple years ago and tried them all. I'm currently using the Astro a50s because they were best, but way over priced (I'm a sucker). The best I tried that were way under $200 were actually the Corsair Vengeance 2000 wireless. I've recently tried their wired H1500 and they are on par. The mic, in particular, is fantastic. I'd go with those: http://www.amazon.com/Corsair-Gaming-H1500-Headset-CA-9011128-NA/dp/B00N803EMU/
Edit: Looks like they didn't adjust the price for GBP and just kept it at 59.99. Jerks.
Bit late but if you go 360 controller route us this I have that type of receiver. The ps3 controller I use wired (haven't gotten a compatable usb bluetooth yet) but people recommend motioninjoy which is terrible SCP is really good and after install should start on pc start up without needing to be opened again, plus it maps the ps3 buttons to the proper placement of the 360 buttons (i.e. left trigger = L2 B = O) and if you use a program like xpadder to map custom 360 controls it works with scp.