The build looks pretty good. I would go straight to Windows 8.1 instead of Windows 8. You can upgrade 8 to 8.1, but you will have to go through that same upgrade every time you want to do a fresh install because the 8 product key doesn't work to activate a fresh 8.1 install.
The windows 8 was a mistake, will change that to 8.1
Would upgrading the motherboard to Asus Maximus VI Hero for ~$100 bring any value, given that I'm not overclocking?
Also considering R9 280x based on previous suggestion. I would be saving about $100 for similar performance.
The 770 is stronger then the 280x unless you are a heavy BF4 player or really interested in mantle. In terms of the motherboard it isn't really worth it for the hero board unless you plan on overclocking although the hero board is going to be build with stronger components certainly.
That is debatable. I wouldn't think so If you were able to get the 280x cheaper. This is how I look at it. The 770 is essentially a supped up GTX680and the R9 280x is a upgraded 7970 which was more in the run of competing with the GTX670.
Although I would argue though that the 770 is not 100 dollars more. I would get the 770 without blinking.