MOBO: Ok but wont alow for much expandability
PSU: I have the same one
CPU: Mid range, will play most games with the right GPU on high settings
GPU: Pretty killer. I have a 270x and it runs ghosts, bf4 at 40fps on medium. this should be pretty decent
RAM: 8GB may not be enough for what you want, but I would stick with this for starters and ge another 4/8gb if you feel you need it.
The changes I would make:
Get a full atx motherboard that allows 2 or 3 way crossfire and 32GB of ram. the MOBO you proposed will not allow for many future upgrades. this brings me to your
PSU. This PSU is not going to allow for another r9 280. if you want to crossfire or SLI, then you will want at least 600 watts (Unfortunately this is the power limit for PDXlan).
Assuming you understand the concept behind the refresh rate of monitors, your monitor will not allow for games to be played above 60fps. If this is fine by you, so be it, but if you are a "hardcore" gamer and plan on playing on 120fps in the future (not that this build could do that given your hardware choices) then get a 75 or 120hz monitor. Just a thought.
UIts a pretty solid build! It is definitely worth the 850$, but there are some changes you could make to save yourself some butthurt in the future. If y6ou cheap out on parts, you will eventually want to get it replaced in the future.
Hope I made some sense, and cheers to the master gaming platform!