What is it you need help with? Your system seems perfectly fine. But I would suggest that instead of the 4670k i5, you should instead go with the 4590, since your H97 motherboard won't let you overclock much anyways (Z97 is required for overclocking).
Only the higher up ones are good quality. The lower tier 500 - 600W ones aren't as high tier.
Honestly, @OP, you only need around 650W for that entire system. You shouldn't buy a PSU that's way too high for your system, since 600 - 650W will do you just fine. I run a similar spec (albeit a 660) on 500W, and I have a friend who runs the same system as you on a CX500. You don't need to spend extra money on a 750W. Buy a good quality 600 - 650W.
The R9 280x is slightly cheaper than the 770. I guess if you wanted to you could buy a 770.
I personally think NVidia is better (this isn't fanboyism don't blame me) because the drivers are more stable. The AMD drivers seem to cause a lot of problems with the higher end R9 cards. This is just my opinion. (Obviously your friend is just talking out of his hat ).
Depends. If the GPU is not a very powerful one like an R9 270, you'd be better off buying a better card vs. sli/crossfire. If you have a 290x, buying crossfire would be useless since you would probably be able to get something better than a 290 for the same price.