If you're bumping up your gaming resolution, then there's no reason why you shouldn't get a better GPU. If you're staying at 1080p, then stick with the R9 280, it's what it was made for. Just my two cents, as if you're upgrading, you should make it worthwhile.
Upgrading to a GTX 970/R9 390 would be alright, saving up for a GTX 980/R9 390X would be better choice, as your CPU is more than adequate to handle it.