r9 280x is a poor value choice
no game in exitence uses more then 4 cores on a processor and i only know of a few that even use 4, most games use threads 1 and 2
that processor will run really hot without an aftermarket cooler.
gtx 770 will give you equal or more performance than r9 280x forr up too 100 dollars cheaper
i5-4670 and i5-4670k are the processors you should look at
other than that though thats a good build