The 280X is a little more powerful but for all intents and purposes it's about the same performance wise in games which utilize multiple GPUs efficiently. The 290X is about 50% more powerful than the 280X, and the Fury (non X) is about 40% faster than the 290X.
In my opinion, and since i own an HD5970, you should get a gpu card that has minimum 4Gb video ram if you plan or continue gaming at 1080p and above resolutions and prefer playing on higher settings + 4-8 MSAA (anti-aliasing) + HD texture packs. Or if you play some bad ps4-to-pc ports...
A 3gb vram gpu card should also suffice for most games, but with >4gb you'll be more futureproof.
Combine that with a powerful single-gpu (avoid crossfire if you can) and you've got yourself a nice playground.
I strongly suggest going for an R9 290X or better, depending on your budget.
Your PSU (power supply) should be enough, both in terms of amperage and watts, with possibly the exception of the R9 295X2.