That mATX motherboard is not compatible with the mITX case. It is too large to fit into the case. Cancel the motherboard if you can, NOW.
Yes, you won't have USB 3.0 available on the front, but the ones on the back of will work.
Oh, I'm not aware of what parts you already own and what's being reused. No need to get the CX600, I was just saying that buying a CX500 for $52 would be a bad idea.
The warning has to do with the fact that H81/B85 is a bit dated and might need to be updated in order to work with that processor. In usual practice, it's fairly safe to buy a H81 or B85 motherboard today unless it's super old stock that has been sitting on a shelf for a very long time, brand new stock H81 and B85 motherboards today tend to come with a high enough BIOS version to support the latest compatible CPUs.
That's already one of the cheapest ones you can buy. Reviews are still decent and mostly positive.
Are there any local retailers you prefer to shop at, or will skinflint.co.uk work for you?
No, Nvidia drivers are all the same. You shouldn't need to reinstall the Nvidia drivers unless there's a problem.
Is there something wrong with your existing RAM that you'd call it shitty? It's cheaper and more practical to add 8GB of RAM to your existing 8GB for 16GB total.
The GTX 770 and R9 290X are discontinued now, neither of them are recommended. Look at the GTX 960 4GB, R9 380 4GB, GTX 970. and R9 390 instead. Or did you mean you were looking at used graphics cards?
The R9 290X would most certainly be much stronger than the GTX 770.
As for the power supply, which one you should get will depend on the rest of your parts, but you should be looking for 550~650 watts which should cover your needs. How many cooling fans and which fans you should get depends on what case you have, the rest of your parts, and what airflow is like.
