Pretty much everything Timmy said is useful and correct, but I must say Windows 8 is not worth the discount. Windows 7 is a lot more user-friendly and functional, not to mention has better compatibility with most software in my experience. 8 is a pain to work with and tends to cause issues you wouldn't get with other 0S's. Save yourself the hassle and go 7.
I'd still say the 4790K is a bit overkill, for just general lets plays and gaming videos you might see a difference of videos being processed maybe 10 seconds faster or so, hardly a difference.
I personally use Windows 8 and prefer it quite a bit, it's got a lot better support for things and has a lot of under-the-hood improvements over Windows 7 meaning that RAM usage will be much lower and it'll boot a bit faster (even with an SSD) for example. You can say what you want about the interface but it's really as simple as just installing a simple program and it'll look just like Window 7 yet you still get all the benefits and improvements of Windows 8.
Personally, after a bit of tinkering and organizing, I ended up preferring the Windows 8 interface over the Windows 7 start menu! The trick here is not to view (and use it) as a huge ipad-like grid of icons but rather treat it the same way you would the good ol' start menu - Just that it's bigger!
Here's a screenshot how it looks for me, notice that it's quite nice and tidy compared to what you may be used to seeing.
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