This upgrade will be a good one, if buying new you may want to consider a 290 as mentioned above. The price difference between a good 280x and a cheap 290 is quite minimal. If a 280x is the only option then yes it certainly is worth it.
performance per price an r9 290 is better than r9 280x if you can afford it. Plus dont forget they are already "old" cards, meaning they are over 1 year on the market, so better go for r9 290 to keep it for longer too
it is a significant upgrade, but i would suggest a nvidia card, they have better features and support, Also they tend to consume less power and product less heat
This upgrade will be a good one, if buying new you may want to consider a 290 as mentioned above. The price difference between a good 280x and a cheap 290 is quite minimal. If a 280x is the only option then yes it certainly is worth it.